In the Talisman UI, right-clicking any unequipped charm equips it to an empty slot
So far, so good
Right-clicking any unequipped charm when there’s no empty slot always replaces the charm in the top slot though, even if there are charms equipped with the exact same affixes as the just right-clicked unequipped one but with lower rolls
In addition, hovering over an unequipped charm shows the item comparison window, with an option to “Compare other slot”. This comparison only ever compares the unequipped charm with equipped charms in either the top or the lower left slots though, completely ignoring the other four charm slots. Right-clicking the unequipped charm while comparing it then again always replaces the charm in the top slot, even when currently comparing the unequipped one with the charm in the lower left slot
What did the designer coming up with this functionality or the developer implementing it actually think that it might provide to players…?! 
The whole equip/replace/compare functionality only seems to work reliably when there’s an exact match between an equipped charm and an unequipped one, e.g. for unique or set charms…