It might be consistent if there is a substantial lead angle - range only increases at the rate given by velocity if there is no lateral movement.
For a 21 degree lead/crossing angle (which might be seen for a moderate parameter, high speed target) then 580m/s range rate is equal to 630m/s velocity.
Or the presented chart has to be revised ... Here a new pic from the tracking STIR 1.8 exactly behide an OSA-AKM at NAMFI with t0 t+6 t+12 and t+17(KILL).
