Bookings - Why am I not allowed to choose timings other than what are available default while mentioning service availability

Bookings - Why am I not allowed to choose timings other than what are available default while mentioning service availability

In Bookings, when I try to create a service and mention the Service Availability - I can see timeslots that can be added are like 10:30, 11:00, 11:30.
Why am I not allowed to add time as 11:25 ?