Experience rewards playing first and winning second. A game played through to the end earns 10 points, each round 3 more up to five rounds, and a win 25. Losing a long game therefore still pays well — deliberately so: a scale that only pays for wins pushes players to leave the table as soon as it turns against them, and it is the other players who suffer for it.
A low total adds a bonus in steps rather than in proportion: under 21 points earns 15, under 41 earns 8, under 61 earns 3. A continuous bonus would mechanically reward the shortest games, when a small total is all the more remarkable for a long one. Each swallowed column is worth 2 points more, up to five.
A table with no human opponent earns half. Playing against bots is a perfectly legitimate way to play, and nothing forbids it — but a table of bots restarts endlessly without waiting for anyone, and without that reduction it would be the most profitable path in the game.
Levels run up to 100. None of them gives the slightest advantage: neither the rules, nor the deal, nor the scores depend on your level. It says how long you have been playing, not how well — that is the question the leaderboard answers.