the exp system is broken

Kills. The sheer number of kills Udyr has is why he's ahead.

Rengar also has the most CS on his team, from what. I can see. More even than the Nasus. Udyr also has a fuckton of farm. Hell, he almost has as much as the mid and top combined.

Getting a bunch of kills and/or splitpushing a lot is a great way to get a big xp lead, compared to everyone trying to group mid, splitting cs/xp and not necessarily accomplishing anything against decent waveclear.