There is more than one possible explanation, but the the simplest hypothesis is that the engine waits till the first frame the skill is "off cooldown" to register the next GCD instead of prorating fractional time. So in that case, the rounding as a computation would only be once (and probably down, since that's just how this game rolls), but there would be effectively a second rounding enforced by the engine not rewinding time.

Lots of games (I'd conjecture the vast majority, considering how many games are for old-school hardware where this was the only way you could really do it) tie input registration to displayed frames; although I know such things because I've been an amateur game programmer most of my life, this can be observed if you play games on a potato like I do and have to wrestle through a few slideshows now and then.

If you lock FPS lower (higher!?) and observe the tiering changing accordingly, then I'd dare say this is the most probable explanation.