Yup, that’s the idea behind the cleaning deposit. It’s just there to ensure that buyers fully clear the locker, and not just cherry pick it. I know it sounds crazy, but it does happen. The facilities who have higher cleaning deposits usually do so because they’ve been burnt by a previous buyer who has left a large chunk of the contents in the unit under a lower cleaning deposit.
Rest assured, the deposit will always be returned to you once you have shown that the locker has been fully cleared.