I can see the point that the sandbags might help to "guide" the AP(C) round into the armor and reducing the chance of deflection, but wouldn't the sandbags cause a HEAT round to detonate prematurely? It had to have some effect, otherwise the crews wouldn't go through the trouble of modifying their vehicles (and often having to scrounge up some "duckbill" trackextenders to compensate for the weight of the sandbags and restore flotation on soft ground).
I also read somewhere that Shermans would also profit of increased mobility compared to the relatively slow German armor, but I guess this has nothing to do with the topic
Addition to what the Major said :
I read in a book somewhere that Patton did not want his Sherman crews to add armor (sandbags or otherwise) because this would mean "that the crews do not have confidence in American made products". How's that for patriottism!?
Also, on Tiger rounds going through multiple tanks, I also read that the rounds sometimes went through the tank without doing real damage!!! I.e. they would punch a hole and go out the other side of the tank, but not destroy it and often not even injuring the crew! And with the wet stowage they would not even cause a fire! They would really have to hit something important (like the transmission or the engine) or kill one or more of the crew, to disable the tank. Guess they were too good, huh?