Hi broken,
as far as I know it seems to be normal behaviour. I'm currently creating a pixel perfect retro GUI where I can see and "feel" every pixel. At one point I had to use MeshColliders for some buttons and I positioned them without offsets. It turned out that the Hover animation for my buttons started earlier or later depending on which site of the button the mouse entered first.
I then added the offsets and everything works fine now.
I still would like to know the reasoning behind those offsets.
Cheers
BrightBit