Soviet mortar = clone of ost mortar = clone of USF mortar.
If you want to talk doctrines, then you have mortar halftruck for poor ost, which will counter 100% all allied mortars with literally a single click while being literally impossible to counter barrage.
And the one stock mortar happens to not be able to move, so its perfectly justified why its got more range.
That information is incorrect the Soviet mortar is different the the other 2.
In addition USF mortar HT is as good if not better than the Ostheer one.