California has the highest population in the US. It would probably be safe to assume that unless percentages skew HEAVILY there will be the highest number of bassists here and the highest number of talkbass members.
My guess as to why "California" got lower numbers than other areas is because the state is so large and contains multiple metropolitan areas that users identify with rather than the state as a whole.
I think the only sure way we're going to know how many members are from California is if everyone who lives there went into their profile and typed in "California" as their place of residence. Until that happens we'll only get approximate numbers.