"waiting times"

I had an appointment for a mammogram at 11am which I was offered at short notice. My notes were available but I was not on the list. My appojntment was at 11am when I checked the receptionist said there were 3 people in front of me and waiting time was 20 mins. It then came up on the board 45 minutes. I eventualy went in for my appointment at 12.10. Why was waiting time so long ?