K... imagine you have a Sableye facing an Arceus.
If the Sableye uses a move of 95 base power against it (not Foul Play) then it's using it's own stats. Means 75 base attack (Sableye's base attack, look it up). By this it's doing about 20% damage.
Now, if the Sableye uses Foul Play instead, then it's using the opponent's attack stat. Means 120 base attack (Arceus' base attack). By this it's doing about 35% damage to the opponent. See the difference?
The higher the base attack is, the stronger the attack is fired. Simply put, the base attack stat determines the damage and is completely different than base power of the move. Normal moves take the user's attack stat. Foul Play take's the opponent's. It's base power doesn't change from 95, but the damage it's dealing does. Any more explanation I think I'll have to do on your wall/chat room now :P