The US Shermans were equipped with cannons that had weaker armor penetration by design so that they could fire more potent high energy rounds that would take out infantry since they were expect to meet very little tank resistance. I think part of the 5:1 myth is from the fact that out of a platoon of 5 Shermans only 1 would be equipped with a gun that could penetrate a Tiger from the front.
Correct. US pre-war tank doctrine was that enemy armor should be taken out with AT guns and dedicated tank destroyers (e.g. M10 & M36) while the Shermans were infantry support and to create and take advantage of breakthroughs to flank the enemy or take out supply and ammo depots. For US units all tanks in the platoon of four tended to be the same, but UK tank corps that had Sherman Fireflys available (a Sherman with a British 17 lb gun stuffed into the turret) would often run one Firefly per platoon since the 17 lb gun could engage and destroy at range any German tank it encountered. This was why the longer barrel on the Firefly tank was often camouflaged to look like a short 75/76mm gun to reduce the chance that it would be identified and targeted.
