On a "bus" the speed bug is driven by the FMGC when in managed mode according to the zero fuel weight entered plus fuel remaining but the Vls speed (lowest selectable speed- V ref) which is displayed as an amber line, is computed by the flight augmentation computer according to the AofA sensors. If you were heavier than expected, the speed bug might be in this yellow arc which would indicate an error, the auto thrust ( if engaged) would not allow the speed to go below this Vls speed. Can't remember what a Boeing does.