I think it comes from the fact that a lot of self-professed Christians have not actually read the Bible. The Catholic university I went to is pretty pozzed, but one of the valuable things they required me to do when I attended was take some religious courses. They didn't explicitly make me use those credits for courses related to Christianity, but that's what I did and thus had to sit down and actually read the Bible for my studies. I considered myself an atheist at the time, and although it'd only be years later when the existence of objective evil was shoved in my face that I'd begin to identify more along the lines of an agnostic or Deist, and years later still after that that I'd become antisemitic, it was clear to me that anybody who actually studied the teachings and devoted themselves to Christianity would come to the conclusion that jews were not their friends.