No doubt, it would have been nice if brainy Katie picked up on this obvious error at the time it was said.
My guess is that he got it from a Los Angeles TV news report a couple of days ago which said that only 48% of L.A. students graduate High School in four years.
http://ktla.trb.com/news/la-me-dropout29jan29,0,4432068.story?coll=ktla-news-1