Some quirks prevent other quirks from appearing, I've heard many complaints about Nearsighted and assume that's one of the reasons it appears so often. The way quirks are gained is changed starting from v2.71. Also, nearsighted gets changed in v2.75 to only affect girls in the backranks.
Additionally, the random generator wasn't 100% random due to a coding error, so that could also have something to do with it.