For Mercenary AAs: Healing Acumen requires is ranks in Improved Spell Resistance, however, if Healing Acumen requires 15 ranks of Improved Spell Resistance, you still can buy a rank in Healing Acumen. You have to raise Improved Spell Resistance 2-3 ranks higher than the requirement level. This likely is prevalent throughout all Mercenary AAs. For Character AAs, I've noticed the same behavior for Foraging AA where the requirement is character level. The level requirement might be 85, but you usually have to raise your character level to 90 or 95 to get the next group of foraging ranks. To get the final foraging ranks for my druid, I had to level him to 105, which is several levels beyond what he needed to be. This likely is a result of rolling up several of the AA lines and not reworking the requirements within the new line.