So do kills give global exp

Top usually gets ahead in levels for a lot of reasons. You're pointing out that you see a higher level person and that typically stems from how snowbally top can be. If a top laner gets first blood, they're primed to stay ahead in lane, meaning they can push or farm the jungle inbetween waves and in some cases fight their laner and the jungler, putting them even further ahead. It's also a solo lane, so they're gaining maximum yield from minions.

This isn't as common in mid because a mid will attempt to roam or be a part of map control in the bottom half of the map and might even trio lane for a little bit to set up kills in bottom.