[Note: This report refers to the question of Jeyne Westerlings hips, described by Catelyn as being "good" for the purposes of having children, and described by Jaime as "narrow". This seeming contradiction has sparked theories that the girl Jaime sees and is told is Jeyne is in fact an impostor.]
I actually asked GRRM about this at the union square signing. When he spoke he said some mismatched descriptions are him doing it on purpose, and some are mistakes. And the mistakes are really unfortunate because it detracts from when he does it on purpose.
When we approached the stage for signings we had the chance to ask a quick question, and he told me that that the hips were a mistake unfortunately.