I'm also SO confused and frustrated by this.
I have a personal friend staying from 7th Sept and checking out on 11th Sept which is 4 nights.
Now, if they'd booked it on Airbnb then those are the exact dates that they would put in to the calendar i.e. the check in and check out dates.
However, when I go to the calendar, as the host, and block those exact same dates it shows 5 nights booked and, consequenly, no one else can book for the night of 11th.
The only difference I can see is that when guests book on Airbnb it's called 'check out' but in the calendar, via the hosts dashboard, it is called 'end date'.
This also results in the rather ridiculous situation that if I want to close my room to new bookings for just 1 single night, I have to put the start date as, say, 27 Aug and the end date ALSO as 27 Aug - now, to me, that means that I haven't booked a night at all, but Airbnb seem to think that I have!