When you run the Availability Report, each box contains 2 numbers, the first number is the number of rooms on that day, for that room type available for sale.

The other number is the sales rate (in line with the selection in the filters on the top of the report).

If the first number shows 2(+1), this means that you have 2 rooms available for sale on that room type for that date, but in real life there really is only 1 room available, but you have manually set an overbooking of 1 room.

Find out more about availability management in this video.