Why are Russian World War 2 mortar bore sizes 1 mm larger than German?

The Russians throughout both WW2 and the Cold war would use rounds a few mm bigger than the opponents. This was extremely useful because while in the field, the Russian troops could use both their own ammo, and the enemies slightly smaller ammunition that would work less effectively. The opponants would be disadvantaged to only using their own ammunition, as they could not fit a mortar round or bullet a few mm too big into their own weapons.