Hmm, interesting. I'll admit to not following my enthusiasm closely enough when I was still in the cup to say. Definitely agree though that PL games when in the Cup were at higher enth than reached during any league win.
Perhaps the speed of decline is slightly faster than what CP estimates, as most values were 11.5-11.9, and would have been rounded up to 12. Easy to see how it wouldn't take much faster decay rate to have those shown as "11" in the lineup/tactics page.