appointment lower limit not working as expected

The appointment lower limit parameter set in the admin seems not working as expected. I set this value to 5 hours and at 12.52 my customer could book an appointment for 16.00. How is this possible ? At 12.52 my customer should have been able to book only after 17.52 (12.52 + 5) Or am I missing something ? Which is the formula applied to check if a slot is bookable or not ?
Thanks for your help.