The reason why, is that the limitation comes into play at the very moment you declare your OB/DB split at the top of the round. . .so before Snap, your OB is 80, not 100, and you must declare how you split that to OB/DB. . .you decide 60/20.
Then, you declare your actions, and declare an 80% attack, which is a -20 OB penalty. . .applied to the 60 as established in your OB/DB declaration, giving an OB of 40.
Even if you endeavored to complicate things, like by mounting or dismounting in snap to attack in normal or deliberate IMO it would go:
Mounting:
Player "OK I take my 100 OB and make the split 80/20"
Player "I mount my horse in Snap"
GM "OK, your mounted combat reduces your effective OB by 20, your OB is now 60/20"
P "I make an 80% attack on Orc 1"
GM "OK, attack at 60-20=40 OB"
Dismounting
Player "OK I take my 80 OB and make the split 60/20"
Player "I dismount my horse in Snap"
GM "OK, your mounted combat reduces your effective OB by 20, your OB rises 20 and is now 80/20"
P "I make an 80% attack on Orc 1"
GM "OK, attack at 80-20=60 OB"
I would generally make the in round change on the OB side, unless said change resulted in a negative number. . .