in 9.9, Wargaming fixed (well, changed anyway) a number of models in World of Tanks. This is tied to a rather big "pewpstorm" that appeared on RU server but did not reach the EU server in general because it was mostly baseless. These "pewpstorms" are generally fueled by Russian videomakers Murazor (and sometimes Jove) and this one was about some tanks being allegedly easy to penetrate due to issues in collision model (especially the bottom parts). Basically, when you were shooting under certain angle, the shots were hitting the bottom and it was easy to damage such tanks. How important changes those were... you be the judge.
The first picture is always the old, the second is new. And thanks Awasaky :)
T-54 Prototype (here we can see how the "step" was removed)
Oddly enough, regular T-54 still has it
Object 140 was not changed
Object 430 (the hull bottom became flatter)