Agents use their read on a person to determine entry/denial. The OP mentioned all the things he was asked and how he answered. He didn't/couldn't explain his attitude, eye contact, how quickly he answered the questions, facial expressions.

You've met people you took an instant dislike to - maybe its their hyper-vigilant appearanch; maybe they involuntarily smile at everything you say making you think they know something you don't know. Things like that can get you turned away at the border.