Flurry of Blows - Double Check; Error or Unknown Rules
 

So I have read a bit that Hero Lab doesn't always do Flurry of Blows correctly, but I wanted to double check to make sure it was an error or me missing some rules.

My build is a standard non-archtyped regular monk using only core and AGP books. So everything is pretty simple. Flurry of blows and my regular unarmed strike start off normal (adds my correct strength, adds the correct + attack, etc.) But were things get weird is when I add power attack.

At level 9, power attack is +4/-2. My strength is 23 (17+2 for levels + 2 half orc + 2 belt) giving me a +6 damage (shown correct). I am wearing also a corrosive amulet of mighty fist (shown correct) and monk's robes (2d6 fist damage, shown right).

No power attack:
Flurry: 13/13/7/7 doing 2d6+6 plus 1d6 corrosive.
When I add flurry it shows 10/10/4/4 (-3 instead of -2) doing 2d6+10 plus 1d6 corrosive + 2 flurry

Why is hero lab making a -3 to flurry on power attack instead of minus 2? Is there a rule I am missing or is it an error (standard attack is showing correct as 10/4). Also what is the "+2 flurry" added to the damage? Is there some weird rule when using power attack and flurry? The 4 damage is already calculated in with the 2d6+10.

So is it a hero lab error or is there rules that I am missing?

Neither, you just forgot some wording. With monks you are not a full BAB class at 9th lvl you have a +6 BAB, however when you flurry BAB = Monk Level so you are getting +3, which affects power attack.