Had a look at your listing @Edward79. On your listing it states 'On Fridays and Saturdays the minimum stay is 1 night.'
I'm presuming that the one day booking was on one of these days.
If you go to your calendar page, select 'Pricing and Availabilty' from the top bar.
Select 'availability ' and here you can edit 'trip length' info. Probably you do have an additional requirement rule listed there that is overriding your two day minimum.