Bosses are busy and forget. Just like you encourage a developer to book their own appointment (see Do you know to make sure that you book the next appointment before you leave the client?), a boss should be the same.
When your boss takes a call from someone wanting to book an appointment there are two options to offer the caller:
It used to be that if the appointment was for the same day, your boss would have to take option 2 to keep the PA informed but now with mobile devices you should generally use option 1.