[Arryn]

Originally posted by Cainehill:
Why 6, 11, 21?

Because empirical testing hints that when enemy AIs look at what provinces are strong or weak, a PD value of 11 or 21 gets interpreted as 20 or 30 (respectively), and thus that extra +1 gets rounded up to the next "breakpoint" of 10 when the AI decides if it has enough strength to take on the defenses. I've tested this a bit. I had the AI attacking a province with PD of 20. I reloaded the game, upped the PD to 21, and then ran the turn again and the AI chose somewhere else to attack. Consistently over more than a dozen repetitions of 20 vs. 21, and ditto for 10 vs. 11, etc. Other people have observed similar behavior. Either that's the way the game is coded, or we're having a mass delusion.