Ah, good point, I missed that. I wonder if that's a bug or intended.
Still, 8% shouldn't have that big an impact, and indeed when I removed the CoE multiplier, mastery got closer to crit, but nowhere near as close as for Redcape. I'll keep digging and run with some other gear sets too.
My guess is that it is a bug, though we will have to wait for conformation from a blue poster before making assumptions.