Not rated on the A330 at all, but a look at the retract mechanism suggests the retraction actuators are 'closed' when the gear is up, and 'extended' when the gear is down. As they are presumably double-acting hydraulic cylinders, the fluid displaced by the volume of the ram inside the cylinder itself is likely responsible for the increase in quantity in flight, notwithstanding the actual system quantity has not changed.
Something else I noticed, as regards the gear shortening mechanism - if this operates by displacing hydraulic fluid from the oleo, then that could also be partly responsible for the increase in indicated quantity, as it has to go somewhere...