WebStore feature is pretty much an alpha stage feature, it has a long way to go to be a polished ecommerce solution. It is mainly driven by my own need to sell add on products and isn't really up to the job of general purpose user friendly ecommerce yet but I make it available since it can be useful to others with similar needs for selling download products. Eventually it will evolve into a more complete solution with help files and a lot more functionality but it is going to take a while to get there.
The items are sorted first by primary then by secondary sort which makes it possible to group things together and change the sort within the group. There is a setting in "Settings" link of WebStore for Product List Grouping which controls whether the product list is grouped by product or by offer and this also affects what you see, so sort rank for offers may come into play if grouping by offers and sort rank for products if grouping by products.
Best,
Joe