I'm assuming you're talking about HEAT-MP, which deals a small amount of "chip damage" when it doesn't penetrate.
Like Knutliott said, the HEAT bonus damage is being phased out and has been removed from all tier 7-10 vehicles. Since you said that you've been encountering this while playing tier 7-9 MBTs, we'll ignore the bonus damage. This leaves us with the "average" damage that is dealt on successful penetration and is subject to a small amount of randomization (+/- 10% on most shells). On some specific shell types (ATGMs, HEAT-MP, etc...) there is also a "non-penetration" damage that is dealt when the shell does not penetrate the target's armour and is subject to the same amount of randomization. In both cases, the damage value given in the shell's info card is just an average value that tells you how about much damage you can expect the shell to do (within reason).
There is a major exception to this, which is uncrewed turrets. Uncrewed take significantly less damage than other parts of the vehicle (it differs between vehicles, but you can usually expect to do 75% less damage) from all damage sources (penetrating hits, non-penetrating hits, and ammo fires are most noticeable).
In the case of the Hunter, the turret is uncrewed which means that any HEAT-MP shell that hits the turret will deal less damage than it would have dealt if it hit the hull.