While searching for some Japanese lamps and fixtures to add to my collection, I have noticed that circline lamps tend to be widely used in many different ceiling fixtures in Japan. I noticed that they often use circline lamps in flushmount ceiling light fixtures, chandeliers, and pendant light fixtures. I wonder why circline fluorescent lamps became so popular in Japan.

Isn't the circline one of the first "non-industrial" shape of lamp capable of high CCT, so suitable for residential or restaurants or so?

I would say because the rather limited space in apartments in which many people in Japan live they rather use compact efficient light sources like circlines than long fluorescent tubes.

I have noticed that most Japanese circline lamps are apparently krypton energy saver lamps rather than full power lamps because I have seen that the 40w circline lamps over there are rated for 38w operation, the 32w circline lamps are rated for 30w operation, their 30w circline lamps are rated for 28w operation, and their 20w circlines are rated for 18w operation, while their 15w and 9w circline lamps are full power lamps as far as I know.

Interestingly, I also noticed that some Japanese circline lamps are sold as a package of 2 identical lamps and also in a single package with 2 lamps of different power ratings to allow for convenient lamp replacement of dual lamp fixtures. In addition, you can also find packages of fluorescent starters that contain starters of different power ratings for that same reason as well.

For some odd reason, even though North America often uses multi lamp circline fluorescent fixtures, there are no packages that contain multiple lamps where each one has a different power rating unlike you see in Japan for their multi lamp circline fixtures.

Michael is absolutely right!  I once asked that same question to my boss at Iwasaki while working in Japan, and he replied that Japanese homes are small, so they also prefer small light sources. 

But Japanese people also like extremely high lux levels indoors, due to the different photometric response of the Asian eye which requires higher light levels and increased blue content to produce similarly natural colour rendering quality as the eyes of people from Western cultures.  That explains the popularity of twin and even triple circline fixtures.  Some of the Japanese circlines have different powers so as to facilitate operation on simple choke ballasts.  For instance they replaced the regular 32W circline by a 30W version having increased operating current and therefore lower tube voltage.  That was a clever Toshiba invention which allowed that size of lamp to be run on 100V mains with a high efficiency choke, instead of requiring an autotransformer ballast that first boosts the mains voltage to higher levels.  About a decade later Toshiba reduced the power of all types by about 2W to achieve a further energy saving.  As far as I know that was achieved by a combination of an electrode change and a slight difference in tube diameter rather than switching to krypton fills, but I could be wrong.  I have a few of those reduced wattage tubes so should really check what is their gas filling composition.

While we are on the subject of Japanese circlines, a small piece of trivia is that Japanese companies often market these under the brand name "Sir Klein".  Klein of course is taken from the German word meaning small, and Sir implies that this is a highly-regarded lightsource.  And of course when pronounced, Sir Klein sounds the same as Circline.  That was I believe Toshiba's original way of getting around what was originally a GE trademark on the word 'Circline'.

From what I am aware of, modern Japanese 30w 12 inch diameter circline lamps intended to replace 32w lamps still require a different starter compared to the 9 inch 28w circline lamps designed to replace 30w lamps of that same diameter.

Japan has many special ring tubes: for example, FCL16, FCL30. These tubes are unique to Japan, so it is necessary to use Japanese ballasts. I think it's probably for differentiation.

I also apparently found out that their FCL40/38 lamps are slightly smaller than European and North American 40w T9 and T10 circlines while the FCL32/30 lamps are apparently the same size as European and North American 32w T9 and T10 circlines, but interestingly, their FCL20/18 circlines are roughly the same size as European and North American 22w T9 circlines but they only run at 18w but should be an emergy saving retrofit for 22w T9 circline lamps on preheat systems.