"a 75 mm M4 would have no problem killing another M4 front quarter."
Not correct. The 75mm APC round was capable of @90mm at 500 yards, and the hull front afforded about the same protection, in fact a bit more than that. Front quarter, often would not have resulted in a penetration, especially at farther ranges. A Firefly vs a Firefly? they would have no problem killing each other front quarter, even out to 2000 yards, but that is entirely a different circumstance.
Same goes for the T-34/76, for the exact same reason. The gun can just penetrate another T-34 at 500 yards from a dead on shot. Armor front hull is @90mm protection, with the APC round being able to do @100mm. Any more angle, or farther range, would very likely not go through. Its not a 75L/43, or a 17 Pounder, and has nowhere near the same power. Using HVAP will obviously help, but even that is no guarantee.
The fact many do not go through indicates both gun, and armor, are modelled correctly, and maybe some hard looking at the gun and armor would be helpfull before the usual assumptions that something is over, or under modelled, which seems to be the 1st thing many do. You cannot expect every round to go through, even if statistically the gun is capable.