Why can't some users modify/delete their own bookings?

A user is considered the "owner" of any tentative or confirmed booking which they themselves have added to the booking system, or of any booking request which they have themselves approved.

For example, if User A makes a booking request, and User B approves it (to convert the booking request into an actual booking), User B is the owner of the booking (not the requestor, User A).

Therefore, if both User A and B's permissions are set such that they can each only modify their "own bookings", only User B would be able to modify the booking request they approved on behalf of User A.

It's also worth noting that "owners" of bookings are users, and not clients.

If an individual exists as both a user within your booking system and as a client, the individual is only the owner of tentative or confirmed bookings which they themselves have added, or of any booking request which they have themselves have approved.

Take for instance the following scenario; An individual exists as both "User A" (limited to being able to delete their "own bookings" only) and also as "Client A" within your booking system. User B subsequently adds a booking for Client A, User B is therefore the owner of the booking. As such User A (despite also being Client A) would not be able to delete the booking if their user permissions are limited to only being allowed to delete their own bookings.