Check your weekly prices to see if it's set to a discounted rate for the week. If it is, and your inquiry is for a week or more, Airbnb will automatically adjust the daily pricing to the lowered weekly rate. So you'd end up seeing a lowered per night rate for booking inquiries of 7+ days.
Another scenario might be that you have a standard rate of $49 (for example) but then in the actual calendar you have accepted Airbnb's price tips, potentially lowering the nightly rate of some nights? Or maybe you've gone into the calendar and updated certain nights with specific pricing?
I bet it's the weekly pricing issue, that seems to catch a lot of people by surprise, so check that first.
Ok, edited to add that I just looked at your listing, so it is happening because you have a weekly rate set.