The “add to my bag” will run a full BiB optimization using that item, so that is going to be slightly more accurate.
The upgrade finder only swaps out the item in the given slot to decide how good it is, so it might not end being worth exactly what is estimated once all your other gear is considered. It would be too slow/costly to do a full BiB on every single item that the upgrade finder checks.
There is no error on the upgrade finder check. It tells you, with 100% accuracy, how much your DPS would change if you swapped out your current item in a slot for the new item.
Whether or not equipping that new item would then result in shifting other gear around to ultimately increase/decrease your score by some amount… that’s a different question that is much more resource-intensive to solve.
We limit the number of items you can add to your bag for the reasons stated before: it is too costly to add many items. We picked 10 as a good limit for speed.
That second screenshot does look like there’s something going on - all the items are showing the upgrade value for Thori’dal. Looks like a UI error. We’ll figure out why it’s not showing all the calculated values.
In our next update I’ll fix that issue with the “add to my bag” list repeating the score for the top item in the list rather than showing the actual score of the other items. It looks like this bug sneaked in after a tweak we made to the UI in the last update.