About GDI/GDI+ coordinate compatibility?

Answer 1

One thing to remember is that most of GDI usually runs on the hardware (i.e. GDI functions map to the display driver that implement some functionality on silicon) GDI+ was supposed to get hardware acceleration, but it remained as a software alone renderer.

Try manually setting a few pixels via GDI+ and GDI and see if they differ.

Perhaps the way your particular graphics card transforms co-ordinates deviates from the way it happens in GDI+