Site full says the website

Last year we were holidaying in Norfork on a caravan club site and friends wanted to come along and join us. They tried to book online but could not because the site was full! I has a walk round and found at least 4 empty pitches. They phoned and managed to book in.

We are now at West Ayton and again according to the website this site was fully booked lastnight. Incorrect I lost count of the number of empty pitches available. 

Can anyone explain why this happens, should I stop booking via the website and phone instead.