This problem existed in the first release of MTW and it was fixed in the patch.

The origin of the problem is the random numbers used to determine "hits" and "kills" (for example, did the archer's arrow kill the horsie?). Each individual battle uses a "seed#" to tell the random number generator (which is really just a list of previously generated numbers) where to start spitting out random numbers on its list.

Apparently the replays are not using the same seed#'s as the actual battle, so different soldiers will die in the replay than in the real battle. This will cause a massive ripple effect that changes the entire outcome of the battle as shown in the replay--because before long you have soldiers who died in the real battle who are kicking ass in the replay. . . .

Anyway, that is my understanding of how the problem is occurring. How and why this was allowed to happen AGAIN by CA is one of the quality-control mysteries of the universe.