It is normal for restaurants to have online reservation systems. It is great for customers to be able to see the reservation status and quickly make reservations, compared to calling each and every restaurant by phone. However it is bad business if these processes do not work together.
I just tried to make reservation online to a specific restaurant, but the online system complained that there were no seatings available. I refused to believe the online system as it is not normal for that place to be sold out, and as I called the restaurant – they had room for us at the requested time without any problem.
That restaurant almost lost a customer, because of their online system.