Not 100% sure of this but pretty confident that is based on the number
of active listings for the product.
If there are a lot of listings for the same thing, at the bottom of the page it says and xx more in ebay stores.
If there are a small number of active auctions it lists the store items at the bottom of the page like a regular listing.
If there are no active items then only the store items are displayed.
Somewhere in the store faq the actual numbers are provided to confirm what I wrote above. I recall reading it all at one time.