Weekend pre-bookings

I've just booked 3 weeks holiday on three different sites in the south of England as a new area to explore  My first 2 options were easy to book for the dates I required  My third week was to have been at the 260 pitch site at the New Forest Centenary site - simple you would think as it is so big but, because my last 2 days were over a weekend and because that weekend there is already fully booked, my final weeks plan has been spoiled I fail to understand how a huge site can be fully booked for 2 days yet most of the rest of the month is okay  I have got in at Crossways instead