If anybody wants to make sense of it, the way I look at it:
Spider-Man wasn’t completely bonded to the Symbiote, so it very much was just an advanced costume. When Reed Sonic Blasts him, the Symbiote comes off easily and Peter is physically unaffected.
Why does Venom have the full mouth? Why when Venom opens wide you don’t see Eddie’s head inside the mouth? Because Eddie and the Symbiote bonded at a level Peter never reached. When Peter Sonic Blasts Venom, the Symbiote doesn’t remove easily and Eddie is hurt with the Symbiote.
You
shouldn’t see Eddie’s head unless the symbiote is hiding in his body, he’s mid transformation or the symboite pulls away from his head…we refer to Venom as “he” or “him”, but Venom has always said they are “We” for a reason. Eddie’s human head no longer exists on its own in a normal way, it’s completely merged with the symbiote in a shared coexistence. The human takes the shape of the symbiote.
I see Carnage as the same.
The problem is other characters who’ve worn Symbiotes and the original logic that was expressed in the comics wasn’t applied (Mac Gargan, everybody that’s been Venomized or Carnagized, etc). It’s become more of an art thing than the original reason.
It also seems to be a form of expression. In ASM #800 when Eddie gives the symbiote to Peter to fight Red Goblin (Green Goblin/Carnage) Peter again wears the symbiote as a costume but only gets the full Venom head with mouth after he’s extremely enraged.
It could be reasoned that angrier characters get the full deformed heads and bond faster…Eddie, Cletus, Mac, etc, all quite disturbed people. Also with Flash Thompson it was established the Symbiotes take on traits from their hosts, if the host is beyond angry, the Symboite bugs out.
