Compensatory Leave on Holiday Miscalculating

I have an employee who is on a flex schedule (meaning essentially 12 hours available for work 7 days a week, and he just enters the hours he works - no set schedule). On a recent holiday, he entered 2 hours of compensatory leave because employees get 8 hours of holiday time and he usually works 10 hours. SAP is calculating 12 hours (full calendar day) of comp time leave. Any idea why this could be?