The sinking or the cabin in a hydraulic lift is a result of the oil compressing in the cylinder (or piston), due to the extra weight (there should be no air in the system). This will be more profound when more oil is in the cylinder, i.e. at high buildings at the top floors. Also keep in mind that the piston travel that you experience is dependent on the [suspension](http://www.mitsubishielectric.com/elevator/overview/elevators/systems.html).
This is "dynamically" compensated by a system that detects if the cabin has moved from the floor level and is activated if the cabin moved beyond a limit.

Usually, the cabin is hold in place by a mechanism if the load difference expected is quite significant (ex. loading by a forklift). If the customer pays, you may find it in smaller lifts, but it is extremely rare.