Awesomenauts XP system: Level ignores XP difference
This has to do with how experience is credited to players. Whenever multiple players would gain experience from something (or at least from a droid, which is what I tested with), the full amount of experience is credited to both players, however, the team only gains the experience once. Thus, sticking together causes scoreboard values to inflate compared to the actual team experience.
To test this, I used a few custom games with no bots in them. By mousing over the experience display, I found that the experience to get from level 1 to level 2 is 135. Using one Awesomenaut, I found that defeating droids and neutral creeps awards 5 experience to both the Awesomenaut and the team.
In a splitscreen game, I moved both Awesomenauts to a single lane and let droids fight each other. Upon each droid death, the scoreboard showed both Awesomenauts as having gained 5 exp, but the team only leveled up when both had 135 exp, not 70 as one would expect if they were both truly gaining the experience. Experience values on the scoreboard acted that way whether I had one of the Awesomenauts shoot at the droids or not.
In another game I tested creeps and found that only the Awesomenaut which dealt the final blow was credited exp, so creeps cannot cause team experience to desync from individual experience. Also, I verified that if only one Awesomenaut is near a droid death then the experience is only credited to that Awesomenaut.
I did not test Turrets or enemy Awesomenauts, but would expect results similar to droids.