OK, well, I guess it's a matter of who you play as, and if the game decides if your opponent should be invincible or not.
I'm Ultimo Dragon, and I can not make a dent in Brock Lesnar, ever. Exception being that if I belt him with a chair 10-15 times, his head goes into the yellow. And I'm not joking.
I also fought the WGTT backstage, an went nuts on them with a hammer. Couldn't hurt them for shit, though I've owned them in real matches. The game just decides that sometimes, you're just supposed to lose.