let's assume there is a 2 hour booking of a service provider at 10am.
Until 10.00, it is possible to book neither the 10am slot, nor the 11am slot (which is correct).
However, as soon as the starting time of the reservation passes (eg. at 10.05), the timeslot at 11am becomes available again! (Although obviously the service provider is busy until the of the 2h reservation, which is 11.59).