FCTM reads:
"Minimum maneuver speeds (displayed as the top of the lower amber band) should not be confused with flap maneuver speeds. Flap maneuver speeds are based on airplane weight, while the minimum maneuver speed is based on airplane angle of attack and current airspeed."
With the flaps up, there is a comparison between the FMC calculated manoeuvre margin and the Stall Warning Card calculated manoeuvre margin and the higher of the two is used.
If you say that the manoeuvre margin changes with different ZFW's, I wonder if this is only a factor with the flaps up?
I wonder what would happen if no ZFW/GW is entered (FMC still powered, as the signal goes through the FMC to the EIUs). Would the margins still appear?