The upgrade that really isn't

I see a piece of gear that is a slight .13% upgrade. I get said gear. I rejoice. I run BiB, but it doesn’t suggest the change from my old gear. If I equip the new gear it tells me to go back to the old gear. Then when I run upgrade it still tell me the new in my bag gear is an upgrade.

Not a huge problem, but is there something I’m failing to see?

The upgrade finder looks at the items in a bit of a vacuum. It can’t do a full BiB solution for each item (because then some other items might change, etc.). So, the value in the Upgrade Finder is a good estimate of whether or not an item will be an upgrade, but whether or not it ultimately gets chosen by BiB can have some fuzz involved.