One of the features of Amazon's new API system is that instead of returning all customers prices to be repriced it returns a list based on their feedback. The sellers who are returned are determined by “feedback grouping” with only the cheapest seller in each of these groups being given to SellerExpress to reprice against.
This does mean that at times there will be more than one seller per feedback grouping of which only the cheapest is returned. You can see a list of the sellers which have been returned by selecting the ? next to Current Price on your Product Screens Marketplace Tab.
NB. This means that when using “Best price” at times you won’t compete directly with the next cheapest seller.