The depletion level is thought out man its like if you have a level 2 super but your half way to a level 3 but you decided to use the level 2 the bar would use those 2 bars and you will then be half way to a level 1 super lol i know that was a sucky explanation
A better explaination would be
Cost of a lvl 1 super = 10 AP balls
Cost of a lvl 2 super = 25 AP balls
Cost of a lvl 3 super = 40 AP balls
Heihachi has a lvl 2 already and it almost close to having a level 3, which would mean he has around 38 AP balls.
Heihachi uses his level 2 super, 25-38 = 13, which brings him down to a level 1 super, plus a little extra on the side.
The numbers are not at all correct, but just chosen, so the meter and super usage would make sense.