Why are UV coordinates different from Blender

[MathemanFlo]

I realized that ArmorPaint and Blender use different coordinate systems for UV coordinates. While ArmorPaint has its origin on the upper left (like it is the case for most image coordinate systems, the 'DirectX'-coordinate system), Blender has its origin on the lower left (first quadrant of a cartesian coordinate system, the 'OpenGL' coordinate system).
I'd like to know why is it done this way? What advantages does it have? Is it a serious bug? At least it means that we are incompatible with Blender in this respect and I think the difference I got for normal maps (#895) comes exactly from this.
I also tried whether it makes a difference to use ArmorPaint with DirectX or OpenGL but I could not see any (which is good imho)