Tristana randomly not getting a 4th stack on explosive charge (0.8-0.9 AS)

So I was trying out jungle Tristana in custom while initiating with E and not using Q to stack it(since I didn't have Q at lvl 1 and and at lvl 2 since I though I would be able to use the extra damage from 4 stacks on W based on my first clear). But what happened is that sometime I would get 4th stack on it and sometime I wouldn't without me doing anything different. At first I though that it could be caused by a difference in the duration of Auto-Attack that would happen to make small AS increase possible, but it's clearly not the case since that wouldn't make such a big difference(if anyone know how many tick per second league is run in I'm really curious now). So what happen is that the time between casting E and Auto-Attacking varies a lot. This really annoying for jungle Tristana and prevent her from becoming a meta jungler... Jokes aside this feels really clunky and it affect lane Tristana just as much since it's often hard to get any stacks on E early on. I do think that she went from "underpowered" to "ok" with her latest buffs but I still think this should be changed because it's... well... clunky...

Edit: I know that this happens with 0.79 to 0.88 AS. I haven't tried doing it with more or less attack speed but I see no reason why I would get a guarenteed 4 stacks with over 0.88 AS.