Hi there, I have this problem too.
The airbnb people told me its because i have a min 2 night setting and the calendar presents only the day you can stay the first night..... So it looks like the niggt before a current bookinb the place is unavailable. But it is, but not as day 1.
Very silly. Im not getting any booking now for a popular weekend because someone booked their night 1 on a Sunday!
How did you fix it (other than changing your entire availabilty to one night stays?
Robyn