It doesn't target the opponent's attack stat, it uses it. I remember that from lurking in the Uncharted Territory thread, and the Data Audit confirms it:
The Pokemon surrounds itself and the opponent with a dark aura, and launches a vicious attack that factor’s the opponent’s attack Rank against their defense Rank.
Foul Play, how you envision it:
(10+(3-0)*1.5)*1.5 = 21.75 = 22 Dmg!
Foul Play, how it's happening:
(10+(0-2)*1.5)*1.5 = 10.5 = 11 Dmg!
Bite, for your consideration:
(6*1.5+(3-2)*1.5)*1.5 = 15.75 = 16 Dmg!