Aatrox 3rd proc on w not hitting on third hit

I was playing aatrox lately and i realized that occasionally, his w did not proc on the third aa but on the fourth. Just wondering if its just me or if it`s a bug. Usually it happens after being out of combat (from enemy champions) then hitting minions. Is it because the aa counter for proc resets after being out of combat? Is it an actual bug? This does not necessarily affect the game in a serious "win or lose" way, but it is annoying. Thanks for any input!