[Imp]

This may be something to do with its very low Pen value but most guns up close with accuracy of 80+ or so give extra pen reasonably often, not sure if I have ever had an over penetration with the Boys even banging away with 95% hit chance, think yes but very rare indeed.
If it did this would near enough fall in with Pen values & report posted by Cross.
This type of shot represents both the square hit it stresses is important & shooting at particular parts of the tank so damage only occurs with a well aimed shot.
It would still slightly under perform but those are ideal conditions when achieving penetration is marginal anyway even a couple of degrees can make the diffrence plus its a light projectile velocity is nothing special so probably more prone to richochet.