Some thoughts on this - the band is Chaos undivided and he is blessed with all four Marks but can:
Only call/activate one at a time.
Is only choosing to use Slaanesh at the time of that battle.
His aspect is random or changes before going into battle, he is chaos after all.
Warp interference maybe.
Just be creative and give a good reasoning for it.
Chaos- Black Legion & Steel Brethren
W:16 D:7 L:9 W:9 D:2 L:5
Eldar Dragon Claws Corsairs
W:16 D:4 L:2
Da Skull Krushaz
W:27 D:2 L:13
W:14 D:0 L:3