I moved into an apartment complex where the key is both unique to each apartment door, but every key can open the front door, how are both possible?

Question

I moved into an apartment complex where the key is both unique to each apartment door, but every key can open the front door, how are both possible?

In: 986

Answer 1

Many common residential keys have 5 or more different positions that move pins various distances when inserted into a lock.

Common area lock cylinders can be configured to only check one of those positions, and as long as every tenant has a key with that position cut to the same depth, every key will open that lock.