AP vs Bunker/Pillboxes

[jivemi]

In scenario #456, Makin Atoll, the US forces have CS and light tanks with max armor penetration of 8 (L31 75mm) and 6 (L57 37mm) respectively. Yet on a number of occasions they were able to take out bunkers and pillboxes with minimum side armor of 12 (or higher) from the flank or rear (where armor is even thicker). The tanks don't have sabot or HEAT, so how is this possible? Thanks.