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In short.... Nivomat or other "adjusting - height" shocks are more firm as they are more of a supporting - weight shock, so the springs are softer.Bingo; it came with manual self-adjusting shocks ("Nivomat" "Sachs") and softer coil springs in the rear.
ZW7 = Premium Smooth Ride
G65 = Nivomat rear shocks with softer springs
Replacing the self-adjusting Nivomat shocks to standard requires also changing the springs or things can get squirrely, as you're experiencing. So either Nivomats need to be re-installed or the springs need to be changed to a Z71 or standard spring.