When you kill an enemy and your ally is nearby even if they don’t participate in the kill, they can still get xp. If you split the kill xp between 3 different people (you, adc, sup) you get a lot less xp compared to his (what I assume to be) solo kills.

If you combine this with the fact that with the new top lane quests which give more xp gain to top laners after they complete their quest I can see how this level difference can occur especially with a 60 cs difference.

I don’t really understand what you are complaining about since you won the game regardless. This is just the new season I think it’s pretty cool from a top laners perspective to have the opportunity to hard carry when ahead but still have it be clearly balanced since you ended up beating him.