I'm not sure if I'll be to explain this right. I have a dual monitor display on Windows 7 and my left monitor (laptop) has a lower vertical resolution than the right monitor (regular LCD monitor). So obviously, if I stick these monitors together, there's a part in the upper-left corner of my LCD monitor where cursor cannot go to the other monitor. I remember there was a trick to make this gap disappear, and make it seem like the monitors are of equal height.
By the way, I am also using UltraMon.