Which is just about as unrealistic as, say, claiming that road leathers will protect you against the bullet that passed clear through your power armor's plating. Or that Heavy Combat armor fits in a frame, despite heavy clipping while getting inside.
There's an element of gameplay segregation going on here. Some of it can be explained, some can't.
My original point was an explanation of why armor doesn't provide additional bonus to armor under PA. Because, realistically speaking, the list of weapons that have enough punch to get through PA but not any additional armor is going to be small, sandwiched between the weapons the Power Armor can successfully defend against, and the weapons that could trash the whole thing.
Essentially, an attack will either be weak enough to be stopped by the armor, or hard enough to blow it to peices. And if an attack can blow it to peices, you're going to be lucky if it lost enough energy to be stopped by any body armor thin enough to fit inside power armor.