When entering an appointment, lasting 30minutes at 4pm

When entering an appointment, lasting 30minutes at 4pm on the 11th of July, i find that the times from 2.45pm-4pm were then marked unavailable. Why is this? I want it to just mark out the 4pm-4.30pm time.

http://borders-lbs-and-ozs.co.uk/wp/?page_id=10&wcalendar=1372636800&app_service_id=1