What is causing such a extrem variaty in power level beside arguments?

Hey,

playing with my buddy nonstop arena. We pretty much play the same champs over and over again. And noticed a strange difference in power levels for no visible reason. There was a ww with near 5.0 attack speed with not a singled AS Items or Argument.

Or Ivern Ult solo Killing in couple of seconds a squishy.

Sometimes The same Yi build does high damage but sometimes it barely does any.

We understand bad runs caused by unlucky arguments, but sometimes with same good arguments It feels like everyone is 10x stronger as we are with random arguments which shouldnt give them any power.

I saw a wiki page where the arena changes for each champ is shown, so we already take that to account, but that isnt changing every match, so we are wondering what is causing such an extrem power level switch.

Sometimes there is a trundle with usual bruiser items beeing useless, sometimes he is killing everyone in 2 sec. with same bruiser items an similiar arguments. It makes no sense for us how this happens.

When we see a strong/bad argument on enemy we understand why the enemy might be strong or weak, but often we see the simliar arguments, items and champions and huge power level difference in other matches.

Any Idea?

Why does for example sometimes a Yi with Reapers Toll, does high damage and sometimes litterly only doing 1% per hit on bruisers.