This could happen for two reasons.
1. Buffer times.
Buffer times are times applied before and after your meeting. This means that if your buffer is 30 minutes and you have a meeting at 11am, Calbot will take it as you being busy from 10:30am until 11:30am. You can adjust or switch off your buffer times on your "My Profile" page -> Preferences.
2. Working hours.
Working hours is the range of the day when you are working. By default this is set to 9am-5pm on Monday to Friday. Saturdays and Sundays have no working hours. Because of these settings Calbot might assume that you are busy and show your schedule as fully busy. You can adjust these settings on your "My Profile" page -> Preferences.