That's for two reasons:
1. Overshooting a balance task. Quite often, a balancer focuses on something that's OP or UP, and fixes that, but goes too far with it, and doesn't have time to investigate new strategies (that should still work, but that shouldn't always work).
2. Adding new features. Every feature added and even some bug fixes (like fixing the infinite chase issue) severely changes the balancing of the game. A19 won't be balanced very well either, as building capturing is completely new (and that determines gameplay for a big deal). It's hard for us to come up with a lot of new strategies, and compare them to each other to see which ones are OP or UP.