I'm confused about how CDR works, particularly in URF

I was playing URF earlier today like any good red blooded procrastinator and was playing zilean, a champion I enjoy, have recently started maining, and thought would be good. His Q is a 10 second cooldown at rank 1, I thought 80% CDR would mean that it would be a 2 second cooldown at rank one in URF mode, but it ended up being 2.6 seconds and I can't figure out why. Is there some strange math that I'm not picking up on here that makes cooldown reduction not as straight forward as I was assuming it should be? Are champion base cooldowns adjusted in this mode so that the level one spam isn't as crazy as it could be? What gives? What am I missing? I thought that with a max rank W on zilean I'd get a 1.3 second cooldown and effectively be able to permastun someone assuming no tenacity since max rank W cooldown is 6 seconds base cooldown, but I ended up with something like a 1.6 second cooldown on W at max rank. Needless to say I was dissappointed at how not completely broken my champion was, and have been wondering if there's some magic math that's in play here that calculates cooldown reduction in a different way than you'd think.