Why are you hung up on what the numbers 'mean'?
The numbers do not 'mean' anything at all other than give translated values from different axis accelerometers and gyros.
It could be argued that the software really doesn't need to indicate these values at all and only giving a mod value for each axis would be a better option - that way you are only interested in whether the IMU's are within acceptable tolerance limits and if not you calibrate accordingly.
The real clever stuff is done within the Naza itself taking these accelerometer/gyro values, translating them into inertial, temporal and spacial data and compensating accordingly to keep our aircraft airborne.
We are not in fact flying our Phantom or Naza equipped craft at all.... the flight controller is. (with the exception of manual mode which gives linear stick control/input).
The whole fact that the values are shown at all in assist is more than likely a left-over from when the engineers were debugging the code and needed a visual representation of what was going on and how the algorithms were compensating against different external forces.
Don't worry about numbers (unless it's your bank balance numbers
) - just go and enjoy flying !!!