Currently all tank cannons have the same level of dispersion - 0,075 (comparable to dispersion of bolt action rifle).
In 99 % cases this level of dispersion is neglible, but it makes a difference when shooting at long distance.
I think that short-barrel tanks (Panzer IV E) should have higher level of dispersion than other tanks.
This will add the game more realism 
AT-Guns always been a bit buggy. Very hit or miss.
But I think you aimed too high (even tho you get the green hit-marker). The dispersion might just have made you miss this one.
I assume you run the gunner perk? (The difference is actually quite big if you run it or not)
Marker does not lie. While it is true that sometimes it might be grey and a player hits the target, a green or red marker is a 100 % certain hit.
Yes, I missed the shot because of 0,075 dispersion of IS-2 cannon.
A green hit marker can be on the very edge of the target area. And if dispersion kicks in, you miss.
That’s what I was trying to say.
Bad idea, DF would just use this an excuse to further nerf tanks.