It's a relatively small gym so I assume they don't have the resources to improve the system and quite frankly I too would prefer if they spent the money on more equipment
Ohh, I see! At my gym, locker keys are given to you by the front desk and you put in something as deposit (such as your gym card or whatever you wish) and on your way out you give the key and you get your deposit back.
Assuming you don't have an automated system to give away locker keys, wouldn't this be explained by the fact that gym front desk is more likely to give out the lowest number available and as you took X, they will give out X+1 for the next person?