I find it very improbably that someone who has 500 levels on a person isn't significantly better unless the lower level individual is an ex Dota savant or something.
Sure, they may have more knowledge. But just because they played a lot doesn't mean they actually improved. I've literally been in the same spot in Diamond for years now, for example. I'm not hardcore enough to parse through replays and so forth to get better and I'm happy where I am.
This game has a high level of variance, as well, which accounts for a lot of the shit/lopsided games we see. Dunktrain touched on this in the last Into the Nexus podcast.
Players don't appreciate the variance in their own play or the play of others from one game to the next. Furthermore, they vastly underestimate the variance from a player who is Diamond playing the 5-8 heroes they are amazing at down to the next 5-8 heroes they play occasionally. In every game there's a myriad of factors from the map to the heroes to the comp that impact how things turn out. Like Dunk says, some are good on certain maps and not others, certain heroes and not others, and in certain roles and not others. This dramatically affects how the game plays out.
Ditto with player levels. The game thinks that you are all in a similar ballpark of MMR overall but that can wildly differ depending on what you are actually playing from one game to the next. The only person in that picture I'd be concerned about is the level 18 -- it's quite easy to ride a run to higher MMR at a low account level before you tail off. I'd say anything sub 50 is cause to be very nervous and expect a wide amount of variance, and for me personally anything sub 200 makes me nervous anyway as I rarely see those people in my games so when I do I know something probably went pear shaped.