So it's more down to the players they have than their playing ability? Or is it a mix of both?
I've only seen videos of bloodbowl, so i have no idea how complex it is.
It's rather complicated, a very important part of bloodbowl is keeping your key players alive, so that they may level up in an rpg like fashion and gain more skills, and therefore become more effective at the game. Obviously both the opposing coach, and the dice now and then, want to see blood. The less players you have on the pitch, the less likely it is to score or prevent a touchdown.
Now both tacticts and strategy are different for each team. They pretty much all have their own rules if you want to play them optimally. Especially in a private league where due to the limited amount of matches you have to be careful how you spend your money.
As an example if you are playing a woodelf team, you might to pass on some risky scoring opportunities and instead focus on being evasive, keeping your players in one piece when playing against a very strong bashy team.
Another thing to consider is Team Value, basically a score given to your team depending on how many players, and how good they are, plus some other stuff such as fame, cheerleaders etc... Each team in the league started with a 1000 TV. Most team now have higher TV because they scored a lot and some of their players probably gained a level or two. The less lucky teams instead probably lost TV due to having injured or even dead players.
When a match starts the TV of both teams is compared, and the one with a disadvantage gets some inducement money; which can be spent on mercenaries, bribes, wizards, or even starplayers, to make the match somewhat balanced. So in theory every match is somewhat balanced.