Hi,
It's just how Airbnb's search algorithm works. If your house is meant to accommodate 7 people and is priced for 7, Airbnb is going to display you much higher in the search rank when a guest looks for a place with space for 7. It wouldn't really make sense for a guest looking for just a room for him or herself to be bombarded with, for the guests own requirements, overpriced and unnecessarily large accommodation.
Perhaps you could set a lower base price for a select number of guests, say a price 4 guests, then add an extra guest charge. This way you may be more suitably priced for varying amounts of guests. However, you do run the risk of some people lying about their guest numbers
Good luck!