Why is there an appointment on my list but not on my calendar?

Point of Sale offers a feature in Setup Services to have an "Add-On Service".  Add On Services are 0 duration, therefore they do not appear on a calendar but they will appear on the Appointment List, Client History and the Checkout Tickets.

To confirm, go to Setup > Setup Services > Setup Service and select the service that does not appear on the appointment book.  Check to see if the "Add-On Service" checkbox is checked.  

Please note, Add On Services are not available for booking online.