Studies have shown that Pz IIIs and IVs were just as likely to cook off as a Sherman when penetrated. Plus, the German habit of continuing to fire until the target burst into flames (since armour distorted by heat can't be repaired) may have given rise to the idea that Shermans were more vulnerable to fire than other tanks. When the Sherman was first produced the armour was perfectly reasonable, the problem was that it shouldn't have been anywhere near a battlefield by 1944. Blame Patton.