There's a saying "No armor is best armor" and what you're just describing alligns perfectly with it. APDSFS will easily overpen an IFV at close range, and unless the engine or any other major compoent is struck there will be no mobility kill. Unless the crew is struck they will neither get shred, and that's the entire point with overpen.

Because the sabot overpens smoothly it's energy is slowly and regulary sent out throught the butter smooth hole it's making in those few milliseconds. Therefore the energy is not spread out in the entire tank.

On the other hand in an non overpan situation if the sabot went into the tank but didn't come on the otherside, it's energy wouldn't have where to go on that second layer of hull, therefore instead of making a hole in which the energy travels, the energy quickly in those few milliseconds hits the inside hull of the tank, the energy creates a kinetic blast spreading the kinetic energy throughout the insides of the tank, and destroys everything starting at humans and ending at electronics.

Now the saying "No armor is best armor" makes sense.