Any time there's a human factor involved sooner or later someone will make a mistake.

The usual scenarios for wake-up calls that I've seen:
Press "Wake-up call" on the in-room phone to get dumped into the automated wake-up call setup where you specify the time.
You call someone at the front desk/at your service who manually enters the information as you tell them the time.
You call someone at the front desk/at your service who jots it down on a piece of paper or the wake-up scheduler and then enters it into the system later.

I bet you they wrote it down on the piece of paper and either forgot to enter it into the system, or fat-fingered a digit and someone a few doors down got your wakeup call