Some schools appear to give the prep schools the early slots, sometimes this is to avoid the Head passing on interview tips to the candidates with later slots.
I only have experience of MTs & Habs and in both these cases it was apparent that DS's interview was a good few days before the local prep school boys were interviewed.
The first few interview days seem to be given to individuals coming from a variety of schools.
We were given 2 days notice for the MTs interview - which worked fine for us.
Interview tips are definitely passed on from one particular prep school. It is the (sad) reality of the system - yes I am feeling rather anxious waiting now and therefore also the tendency to feel ever so slightly bitter!! But then arguably I guess thats one of the reasons parents pay to send their child to a prep school....?