If you go to a LED fixture the spec sheets say "Utility Wattage Label" and it will say a number 20 as an example, there is another side called "System watts” and it will say a specific number such as 16. Do these companies just round to the number or have to have an even number?

The "utility" label has single purpose: To quickly identify an equivalent system.
Equivalent does not mean exactly the same, but practically doing the same job.

So between 16 or 20W is just 20% difference, that is about the same as tolerance between LED bins (so how much the batches differ in output from each other) or just different bulb make and models of HIDs, so it makes sense to label them as the same "utility rating".
Making too many finely spaced "utility ratings" would cause problems with many people hesitating to use e.g. 20W system instead of an 18W one, so the rounding makes the thing way more practical. So if a LED system in "20W" rated light fails, you (as a repair technitian) throw there another "20W" system and could be sure it should do the job well, without caring that much about what wattage difference can and what can not be accepted (once you do not have the exact same make and model replacement anymore).

So all the wattages are just rounded (according to some exact rule) to one of the few wattage rating number, each will have its light output parameters in some catalog (for correct specification for new installations - that catalog then takes into account the range of what still falls into the same rating).
Plus for the future the rating will mean a certain light output and not that much the real power, so as the efficacy of the LEDs will grow, the rating will stay same for the same output, but the real power consumption will go down, yet you will know if some lantern had "20W" rating, the correct replacement would be another "20W" rating, even when the real power would be e.g. just 10W at that time.

When I was a utility contractor, we had a project where 175 and 250 watt mercury lamps were being changed out to philips retro lux lamps. The NEMA labels we used were green color and the 150 watt conversions got a green “15” and the 215 watt units got a green “22” the retrolux lamps  for the 250 watt conversions were 215 watts but sometimes we used an Iwasaki “super sunlux ace” lamp, it was a 220 watt lamp. The new LED stuff just confuses me, I’m an old school HID guy.. still love my mercury vapor lamps!

When I was a utility contractor, we had a project where 175 and 250 watt mercury lamps were being changed out to philips retro lux lamps. The NEMA labels we used were green color and the 150 watt conversions got a green “15” and the 215 watt units got a green “22” the retrolux lamps  for the 250 watt conversions were 215 watts but sometimes we used an Iwasaki “super sunlux ace” lamp, it was a 220 watt lamp. The new LED stuff just confuses me, I’m an old school HID guy.. still love my mercury vapor lamps!

Interesting, I do know some manufacturers (American Electric is one of them) used to have an option for self igniting lamps on fixtures like the 115. From what I remember, they would have used green NEMA tags too. Do you happen to have any pictures of the tags you used?
Welcome to LG btw. 

When I was a utility contractor, we had a project where 175 and 250 watt mercury lamps were being changed out to philips retro lux lamps. The NEMA labels we used were green color and the 150 watt conversions got a green “15” and the 215 watt units got a green “22” the retrolux lamps  for the 250 watt conversions were 215 watts but sometimes we used an Iwasaki “super sunlux ace” lamp, it was a 220 watt lamp. The new LED stuff just confuses me, I’m an old school HID guy.. still love my mercury vapor lamps!

Welcome to LG!

I’m just surprised with all the crazy different wattages with LED that NEMA hasn’t gone to lumen ratings for tags, I figure it would make for some consistency with judging new installs or replacements. LED tech is still changing and getting more efficient all the time. Who knows, a 50w system right now could be replaced by a 35w one in the future, etc. then what do you do since it’s not even in the same class anymore?

I’m just surprised with all the crazy different wattages with LED that NEMA hasn’t gone to lumen ratings for tags, I figure it would make for some consistency with judging new installs or replacements. LED tech is still changing and getting more efficient all the time.

Well, not all what makes sense is accepted by the masses.
There was one similar attempt: Few years before 2009 incandescent ban in EU there came a legislation in force requiring the lumen output rating to use the largest and best visible font on the lamp packages. An engineer would say why it took so long. But the masses response: "What a nonsense, how should I find the wattage?"
So I bet this was the same story, just the "voice of masses" was heard this time and the rating published in watts rather than more logical lumens.

Who knows, a 50w system right now could be replaced by a 35w one in the future, etc. then what do you do since it’s not even in the same class anymore?

It is simple, it will still be "50W" rated... And yes, it will be the same class, because it will have the same output...

System watts include ballast/driver losses, and NEMA tags only show the lamp wattage. NEMA tags for HID street lights require you to add a zero to find the wattage, except when the tag says 3 or 17. I think LED tags don’t require you to add any numbers and just show the full thing.

System watts include ballast/driver losses, and NEMA tags only show the lamp wattage. NEMA tags for HID street lights require you to add a zero to find the wattage, except when the tag says 3 or 17. I think LED tags don’t require you to add any numbers and just show the full thing.

The only ones I’ve seen with adding a zero is the Philips Hadco “RX Series”. But yes, LEDs usually don’t need you to ad a zero or five.

I wonder why LEDs are different. They should have just kept it the same for all types of street lights.