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

The front door has less pins in the lock. The key goes in and only the end few cuts on the key are needed. Then when you get to your door the lock has more pins and the key goes in farther and uses more of the cut pattern on the key.