I've also had this happen to me- driving from Boone, North Carolina to Charlotte, I usually avoid highways because the interstates are a bit overwhelming to me. I took one different turn as I was trying to avoid downtown in a rural city I was driving through; they were having a block party and the usual route was blocked off. It's routed me through that turn ever since, so saying Google is learning from where people drive even enough to start making routes, that doesn't seem too farfetched for me.