I had the same problem and asked about it. It happens when the nightly rate is different on the two days. The total is correct but the nightly rate is the average rounded up by the nearest dollar, so it may not be correct, but the total is. Other times is because of the exchange rate if the payment is in a different currency. I hope this explanation helps!
Airbnb will average out the cost over the the twelve days. Let's say for example your rate is 43 on Monday, 34 on Tuesday and back to 43 on Wendesday, Airbnb adds these together and then averages the total to work out your nightly rate, in this example the rate would be 40.