daniellyal,
It's not a motion problem, it's an error in the displayed value of the acceleration expressed in G units. When you enter a value for acceleration on the motor tuning page, the box to the right of that should display the acceleration in units of G. If you enter 300 units/s/s and you are working in metric units (mm), the displayed value should be 300/9800=.0306. Nothing to do with pulses, steps per unit, maximum velocity, just a translation from units/s/s to G.
In M3, you can enter either G units or units/s/s but, obviously not both as they are not independent values.
G values are nice in that they give one a quick feel for the forces acting on the table and any vices, work, jigs mounted on it. An actual acceleration of .77 would require very high forces be applied by the motors.
Peter