I've been noticing lately that search results will sometimes vary (store and core) upon refreshing the page or returning via the back-button.
I can't remember for sure if I've seen this in Current lisings but I've definitely seen it in Completed. Do Store results show up separated in Completed? If they don't, then I've definitely seen it in Current listings too.
Yesterday, I searched in Completed for a particular item and there were three results. I clicked on one and when I hit the back button there were only two results. I hit refresh and the third returned.
So you never know what you're going to get.