"Appointment confusion with Sentinel"

I had a phone call last week advising me that I'd been referred for an appointment with a dermatologist. I was asked which hospital I would like to attend, and I requested Derriford, as it is local to me.

I was told I would receive a confirmation letter, but having not received one a while later I called Derriford Hospital to check the time of my appointment. The lady I spoke to said my appointment had actually been made at a very early time at Tavistock Hospital! I would have missed this appointment, and may well have been taken off the list, had I not called to confirm myself. I'd like to know how this could have happened?

The lady I spoke to at Derriford then transferred me to dermatology outpatients, who offered me an earlier appointment. Why could Sentinel not have offered me this in the first place? Instead I was initially offered an appointment not at the hospital I'd requested, and received no confirmation letter.