Some appear to hit, when they really don't - tank shells arn't the size of the tank, what appears to be a hit, might instead be a glancing blow off the armor, rear shots are most common with ricochets, or bad angle shots.
If you film it - you can review the film at a slower speed and see exactly where you hit, more often then none - you either glanced off the armor, or completely missed, however the hit sprites do happen, even if its not a kill shot.