I personally think that AA7 and AA9 flatten primers more than other powders because they are such small, fine, sand-like particles that get down in the primer pocket and flatten them. Try using case expansion instead of primer flattening to guesstimate pressure next round. Just an idea.
I have heard this before, maybe it was from you, or someone echoing what you had said, I don't remember.
it does sound reasonable though, that stuff (AA#9 in my case) is super fine.
To the op:
I have not found anything that meters too much better than Longshot in my LEE powder thrower, and its great for velocity.
AA9 clogs up my Lee thrower because it is so fine, YMMV.
Go with Blue Dot if you want fireworks, that stuff is just too fun!