Rating is ELO based (which is nice), but points calculating depends on amount of points which players had when game was started. I think more fair will be to take some avarage of players points from start to end. For now we can imagine a player which will win season ladder with just 1/0. How? In season X player will start 50 games having 1000 elo. He will lose all. Then before season X ends he will start one game having nearly 0 elo. He will finish this game winning just before season X+1 ends. Not sure how much points he will get, but I think bonus pool will be over 200 and I can imagine he will get about 500 total. Its even possible to happen not intentionaly - just weak player lost 50 games and stopped playing. Then his even worse friend will ask him to play after getting to know he also played this game.
In general players can manipulate - for example keep lost games to the next season to maximize score in current. Then surrender in next season beginning to get lower elo. Then open new games with about 900 score so they will get more points for each win. Then play lazy in games where he can lose and keep it for next season. For example playing 30 games with 80% win ratio per season, starts with like 0-5 and finish with like 24-6 and have like 200 points more than if just played "fair".