Early evening arrivals

We arrived at 19.42 at Commons Wood site, having rung the site to say we would be arriving at that time an hour previous. We were met by the warden who had the key and a site map but we had to pay cash, and the exact amount, we did not get a receipt, and we did not have the option of paying by card. My understanding is that we can arrive anytime up to 20.00 so why is that the sites seem to cash up and shut the office an hour earlier.