(0,0) is the top left corner, so +x is right and +y is down. This is no bug and happens to everyone (unless you overwrite the methods).
Basically what Super_Hippo said. The Y-Values increase as you move down from the Origin (Top Left Corner). Therefore adding one speeds up this process, and moves it down further. And Vice Versa.
Actually, I'm curious what you expect this to do? (Did you mean you would expect it to cause an actor to move up)?