could be the old "rounding error" of sorts we used to deal with in XI. The percentages weren't actually percentages, but a number divided by another... think it was x/1024 if I remember right. The help text would mention the flat percentage that it actually rounded to, but it wasn't entirely accurate. Like 5% haste would actually be computed as 50/1024, but was actually 4.88%, which rounds up to 5%. Doesn't sound like a whole lot, until you are stacking them and what you thought was 25% was actually falling a hair short of the mark.

Don't know if that is how they are doing their math this time around, but they've been known to do it in the past.