About estimating duty cycle

Hi there, I have a question about On duty about step-down converter.

According to the basic principle, on duty is estimated like Duty=Vout/Vin, but the results of the measurement result is a bit different.

I'm using ADP2301-Eval board and checking ON-duty like the attached condition.


But the calculated value and the measurement vale is not nearly equal.

Could you give me some advice on the reasons ?

Best regards,