Apparently, //// Our systems and/or staff will manage the overall capacity to ensure there is a bay for you, which is how we guarantee you a space /////
A regular pre-book wan't guarantee you a specific bay; if you want that, you have to pay for premium, or something similar.
Have a read here:
Not sure about NCP but Faro I booked, car in front of me was denied entry, driver told me car park was full. I put my barcode on machine and was let in and found one space