You would have to assume others sold less than nothing
Seriously, in the way I think they do the calculation, I imagine someone else has added to the 'Other Collections' element of their store, and not sold anything ( or very little), thus making their items sold as a % of their items available for sale less than yours.
This might be totally wrong, but then again, as I have often said before, the calculation is a mystery to all mankind.
I have always had a real dislike for the system for calculating the ratings.To me a top store should reflect the total value of sales in a month,regardless of how many items are listed. I suppose a seller who has 1 item and sells it should be the top seller????
I just ignore the ratings...they are totally misleading for buyers.