Power up bug for my Dragonite (Updated)

My A15D13HP15 Nite was level 37 (on the left) and it should need 12 candies to power up. But the game shows 10, and in order to test it, I transdered my 9 dratini (which I wanted to transfer during the next double candy event).
You know what, it works, I only used 10 candies to make him level 37.5 (on the right).
So have you ever seen a bug like this ? Just curious ^^