It's been the case for a long while that the equipment comparisons in trading screens are for the character that initiates the dialogue, rather than the currently selected one. I agree the latter would be more helpful.
I can't imagine Larian aren't aware of this being the way their system works, but I'm not aware that they have said they're going to change it and it's not something I think I've ever provided direct feedback on myself.
Unless someone else here has, it maybe couldn't hurt to report it and see what Larian support say about whether a change is planned for full release.
