How can you subtract gravity from an orbit and end up with a circle?
If you have no gravity the particle will simply move in a straight line, and that's not a motion I would call an orbit.
If you have no gravity the particle will simply move in a straight line, and that's not a motion I would call an orbit.
During an orbit, gravity is of course stronger when you're near the planet, and weaker when you're away from the planet. So they equalize the gravity by subtracting the relative time/gravity differential and calling it a separate dimension.
This leaves a constant gravity force and velocity, making certain calculations and transformations more natural in this model.