Several items have different stats depending on what slot they are put in. which is displayed sorta like this
(weapon) +xx% attack power
(shield) +yy Max shield
Howerver when you are putting parts into your exos it only displays the stats that add to that slot in the part comparison screen, so if you are putting the above into the weapon it wont show the (shield) stat.
I would love if it showed the stats that don't add in too and just had those lines greyed out, or maybe in red to show us those stats aren't active if placed there. The reason being that maybe the part has just slightly the highest attack power, but by far the highest shield boost of available parts. But when placing it you don't know you are giving up that shield boost by placing the part.
As a note I am not running the 1.001 beta, so I don't know if the new equipment placing interface solves this, but if id does not please consider listing the unused stats in the item comparison so we can know the opportunity cost of placing the part there instead of in a different slot.