When you up in a tank it shows you what the value of the tank costs - it could of been 16 perks at the time you upped and dropped down to 12 perks when you died.
There are times when its 12 perks and you get 40 kills and die and still lose 9 perks or so, it really depends on the type of tanks you kill - T34/40 and 4F will not yield any perks for a Panther, you can literally destroy 100 and still lose perks if you die.
Simply because a Panzer F and T34/40 should not kill you in any tactical sense - however if one does, he will get a boatload of perks for it.
In this screenshot being you lost 16.01 perks, I would guess you upped a PantherG around 17 perks, got a few kills and dropped down to 16.01 when you died.
Edited:
It does not matter what the cost in the Hanger is - when you spawn it will tell you the perk value of the tank you took, it might be 15 in the Hanger, by the time you spawn it could JUMP or decline. However this value of perks is what is TAKEN out of your hanger total perks.