Why you relamp once per year tues that can do 5+ years ? O_o
There is quite a difference between "can do", "do in the average" and "do with less than 2% failures".
The rated life is for 50% failure rate. Good to estimate how many lamps to purchase for spot relamping scheme, but completely unusable for systems, where the relamping needs separate shut down of the site, so where you need to plan group relamping schedule so, there is not too much nonworking lights before the next relaming event. Typically a 2% failure rate is that limit.
If you look at the typical mortality curve of the fluorescents, the 2% failures occurs at less than half of the rated life, so with 24000hour rated tubes you should plan the next relamping latest after 12000 burned hours. And when it means a shut down of the site, you have to plan that to happen together with the existing regular shut down evens (for the machinery maintenance or inspections, factory holidays,...).
Plus there is a big variable in the game: How is the lamp operated? When it is operated 24/7, the 12000hours means a bit more than a year, so once a year period is just right...