Relic have been extremely vague on what AT Nades parabolic trajcetory actually means.
We know there is a chance for it to hit rear, instead of front, when thrown from front, but they havent said how that is determined or what the chance is. Add to that, that Faust can impact on blocking obstacles if the vehicle dodges. ATnade will ALWAYS hit, no matter what.
The superficial chance to hit rear armor, when thrown frontally, is pretty serious though, because it negates positioning.
The parabolic trajectory basically means that the grenade is thrown in an arc, similar to artillery shells fired from howitzers
This means that the chance of hitting the rear of the tank all depends on the position of the tank as well as chance. If the grenade happens to land on the rear of the tank, it counts as a rear hit. So that means that if a squad is in front of a Tiger tank facing forward, and the grenade goes over the front half of the tank and lands on the rear half, it will count as a rear hit
Grenades probably always hit because they are set to phase through most objects when thrown, in comparison to pazerfausts which do not. I dont have the data right in front of me at the moment though so this is just based off of experience with how COH works