This happens if you have added less estimated hours than expected for the task. By default, the system takes working hours as 8 hours per day.
For an instance:
Let us consider you have assigned a user for the 10 days. So the estimated hours will be considered as 10 times 8 that is 80 hours. But if you have entered 40 hours as estimated hours the task, then the resource will be booked full for 5 days, remaining 5 days will be marked as available.
Please refer the images below:-
You can find here User Sean is available from 16 Dec to 26 Dec. We can assign him a task for this period.