I've seen the pilot episode for the first time, and I was surprised that it begins in a sperm bank with Leonard saying that Sheldon usually masturbates like a pro. In the later seasons, he is characterized as the type of asexual person that doesn't do that. He also seems to be more sociable.
It shows signs of the characters being turned into caricatures in the later seasons. Same thing happened in Two and a half Men. In the beginning, they were more or less believable people, but the more the show progressed, the more they got reduced to very simple character traits.
Sheldon turned really strange, really fast. In the second episode, he broke into Penny's apartment to clean, and in the fourth he went of the rails after being fired. I'd say the pilot is the fluke.