Units vs Pixels

I'm attempting to port a game into Torque X and have a concern over pixels versus the units (?) used to represent size and position. If I'm laying out a game for a 1024x768 resolution, I need to use a width of 200 and a height of 150. Why is there a 5.12 ratio? How do you compensate for positions that convert to floating point numbers that go beyond 3 points of precision? If I want to place something at an approximate pixel position, I can't. I can come close, but not precise. Is that normal?