A question concerning rammus when he is taunting tristana. (hard cc and mobility spell clarity pls)

I don't know if it's a bug, if it's fair, or if it's not fair so I'm hoping the community can give me some insight on this. Numerous times have I taunted a tristana (seen the marks above her head, heard taunt go off, went on cooldown) only to have her still fly away with her rocket leap skill. Needless to say, it pains me to have a taunt somewhat wasted. That's a taunt I could've used to lock some other champ down or to peel for my squishies. On the other end, I understand that trist's skill will also go on cooldown and put her out of the fight for a second or two as she successfully timed that leap. Keep in mind that this thread is just for the sake of clarity. I'm having trouble understanding how a hard CC skill such as taunt and a mobility skill such as trist leap can both activate at the same time. Certainly, there has to be a fraction of a second where one person got their skill off sooner, hence taunt should've locked her down or she should've just made a clean leap without taunt going on cooldown. I'm pretty sure this happens between other forms of hard CC and movement skills/ flash. I for one have once magically timed a Diana ult onto someone who used flash to wall hop and I flew right through the wall with em ( only difference being there is no hard CC involved there). Any thoughts? I apologize if I wrote way too much. I probably could've conveyed my thoughts with less words. :)