Not an AE user but I have seen many posts here talking about AE needing to be set to 23.976 which is the actual frame rate that is abbreviated as 23.98.
AE however will give you exactly 23.98 if you set to that and the speed is therefore out with the FCP rate (which is actually 23.976). I presume you are matching field settings (upper/lower or progressive)
Confusing? Thank the slack broadcast engineers who introduced the whole.1% debacle last century. I mostly work in 25fps and such issues are happily avoided.