[jp10]

While it is not directly simulated in the game, the "host of other random factors" can also be envisioned to represent that even if a fired round does not penetrate a bunker wall, that wall will have taken some kind of structural stress. After repeated hits it is plausible that the protection would fail no matter the listed armor value but only as a random unpredictable event.
Which reminds of of the famous M1-1abc 90mm "Knock-Knock" round. By the late 1970s the M67 90mm recoil-less rifle was regarded as marginal in bunker busting the new generation Soviet type of defenses along the Iron Curtain. The M1-1abc round was developed as a two stage wooden projectile in a sabot. the gunner would load the M1-1abc round first, ahead of a M371A1 HE round and aim for a weak spot such as a door.
The twin wooden projectiles, being lighter would travel faster after discarding their sabot and arrive at the target before the HE round. Inside the bunker the enemy would hear "Knock Knock" and when they opened the door to see who it was the HE round would come in.