Generally that is the way it works out with the physics and such too. With the use of armour piercing rounds against armoured targets, the first round or two generally weaken the armour to a point that any subsequent rounds placed near to it will penetrate quite easily.
The round itself is actually a bit of a modification on your standard HEAP round, the explosive used in the way a hollow-point would, to cause more internal damage, rather than to clear a way for the penetrator. Designed to use against infantry wearing armour, not vehicles or heavy barricades.
Either way, the round would strike the armour of the Predator and mushroom, triggering the high explosive core and creating a small explosion, maybe even capable of punching a nice round hole where it struck. If not, the next round most probably will. After this, the penetrator would push into the body and detonate, causing a rather large amount of internal damage. The round could be triggered by hitting the body, or striking the inside of the armour from the other side as it passed through. It all depends on how fine the fuse is set and how much pressure it requires to activate.
Just as a point on this internal damage, the explosion would cause the steel penetrator to fragment, effectively creating a dum-dum round that scatters the pieces farther and with a lot more force behind them. I seriously would not want to be hit by one of these things.
By the way, I'm not actually biased towards the Marines, and I am not just one of Sam's lackeys that listens to his every word and blindly obeys. Apparently I'm a stubborn bastard and damn near impossible to get to do anything. I actually quite like the Predators, loved 'em since I first saw the films years ago, and I don't really mind the aliens. Scared the shit out of me when I first watched Alien with dad at 11pm when I was 7 or something, didn't sleep very well for the next few days after that. Offtopic...
Back on topic now. In my opinion, the round would be quite capable of penetrating Predator armour. It would quite easily kill a marine if accidentally shot at them too, and an alien's skin should be penetrated.
The only issue with the armour of the Preds is that we have no statistics on its actual strength, penetration resistance and the levels of energy it is capable of absorbing before a hole is blown in it, and must therefore speculate a bit. Descriptions in terms of relation to the strength of steel are not accurate either, as steel is an alloy and can contain many different levels of carbon, chromium, nickel, etc, all of them altering the strength. There is a fair difference between 1.8% High-Carbon steel and 420 Grade Stainless. Maybe I'm just complicating things.