It took me a while to realize that they are talking about the number of birthdays listed on the page about March 27[1], not actually combed from all Wikipedia person pages, as I expected from the title.
To anyone who knows even a little about how Wikipedia is edited, this shouldn't be a mystery at all; there are no automatic pages or statistics, even for fixed entities like dates, and there is no automatic cross-referencing.
So as you'd expect, a person entered every birthday on that page, and there is absolutely no reason to assume that comparable detail has been given to all other days of the year.
(I'd hardly call this an easter egg, and I wish more people understood Wikipedia enough to make this entirely uninteresting.)