I get what you are saying now.
So the entire concept of Best in Slot is pretty “fuzzy” these days compared to say, back in Classic. There are a lot of variations of items, so it’s hard to make one single Best in Slot list. That’s why we give you so many controls – you can filter things and customize it, etc.
Thus, the chance of ever getting a single “Best in Slot” set of gear is very small. You will almost always adapt to whatever drops you happened to get. The Upgrade Finder is more of a tool built around a “greedy” approach to gearing: put on whatever is the biggest upgrade for you right at this moment. More often than not, that will lead you to a set of gear that is close to optimal down the road, and it will improve your short-term performance the most.
That said, there are certainly exceptions to this approach. For example, you might want to choose an item with a domination socket so that you can get the set bonus. Same idea with tier sets that will be added in the upcoming 9.2 patch – even if the item itself is weaker, you might keep it so that you can complete the set bonus later on.
Those exceptions aside… it would be pretty rare for an item that is a pure “stat stick” to make much of a difference down the line. If you want to give me a snapshot and let me know the specific item in question, I can take a closer look at your specific case. Instructions on how to do that in this post: