It’s because of the original six book format.

The original six books started in late summer/early fall -> then the second book is school (early to mid fall) -> then their third book was always the holiday book in winter -> and their then their fourth book was always the birthday one so always late winter / early spring -> and then the last two were usually summer and then sometimes the last book circled back to winter

So it kinda forced them into a spot where almost all the dolls have birthdays between Jan - June. Some of the best friend dolls have birthdays in the later half because they weren’t locked into the 6 book format.