Check out the A330 FCOM
Landing Gear section, on page 3 it shows the extended and retracted views.
As regards the
why, this image below shows it at its' most basic. Here we have a double-acting hydraulic ram. The shaft itself displaces a certain amount of fluid when it is 'retracted'. The A330 uses the retraction actuator
extended when the gear is lowered, or in the case of this image, pushing left-to-right.
To retract the gear, the shaft would move right-to-left, but in doing so, even though the ram has the same internal volume on both sides, the shaft itself displaces hydraulic fluid, as a function of its' diameter and how far it retracts into the cylinder. It's likely this displaced fluid is the cause of the increase in indicated quantity with the gear retracted.
