You left out a very important word in that definition. Assumed. The root of the milk epd is a figure that can't be explained. We then apply several assumptions to this mythical figure and hope we're right. It's interesting to note that once we run genomics on females they often can't live long enough to even prove this scientific wild ass guessing game right or wrong. Especially if the given association isn't a whole herd reporting system. 

The Angus association is still operating under a system that essentially financially punishes breeders for turning in data, so the majority of the data that gets turned in by breeders is so incomplete it causes an endless loop of extra assumptions. Fun times.