Even if you don't go to church if you were raised going to church and then stopped, you still might call yourself a [cultural] Christian.

Also being atheist has a bad reputation attached to it for some people, so someone who meets the definition might not self identify as one.

Similarly I expect that's also why there are a fewer percentage of Democrats than there are Republicans. I may have voted down ballot for only Democrats, but am I a DNC supporting Democrat? Not really.