I have the same problem, and I have been trying to get it fixed for ages but no luck! I think they set 'extra guests' at a low season rate, so if you put your rate up in the high season, it only goes up for one person. If more than one person books at the same time, airbnb automatically use the extra guest rate which is usually a low season rate. that is calculated and divided and they get a rate that is wrong. We have to keep on explaining to our guests why the rate is actually higher and lose loads of reservations because of it. Airbnb doesn't seem to have the time or motivation to fix the problem, and they don't own up to it either.
Another problem is, when you send the person a 'special offer' trying to rectify the wrong rate they were provided by airbnb, airbnb hike up the rate and put an extra amount on top of your price, charging the guest more and paying you less. This is not good either. I can't get to the bottom of this, it's very frustrating, because I like airbnb and the people who reserve through them.
If you get anywhere please let me know.
Thanks, Sandy (HOstel & Patio Bar Mais Que Nada, Bahia, Brazil)