Just to help you (and those will be able to give advice), could you confirm
A. the part numbers you ordered.
B. the part numbers that are on the two boxes
C. the part numbers on each airspring
D. photo of the air springs

I successfully used Arnott rear air springs on my car (rear only self leveling), so it might be that you have one correct one and one incorrect one - they are both the same length / size for an E53.

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2002 Euro X5 (E53) 3.0 diesel (M57)
5 speed manual with UUC short shifter.
Pre-xDrive NV125 transfer case (chain & gears).
Factory Xenon headlights, Spyder LED tail lights
Updated vortex crankcase breather.
Motorsport dead pedal, factory fire extinguisher, factory first aid kit, factory F&R mud flaps, factory PDC

Bilstein B6 shocks F&R
Redline oil has replaced "lifetime" fluids (F&R diff, manual transmission, transfer case and P/S).
Dimple magnetic drain plugs in all compartments.