The uplifted sundered are not actually given their real memories back.
So they are being given artificial "memories" that were made up by Emet-Selch, Elidibus, and Lahabrea. And the Unsundered didn't live the lives of the Sundered; they only saw parts of their lives from an outsider's point of view. You don't live your friend's life, do you? If he had amnesia, you could try to tell him what you think he was like and what he did, but you weren't with him 100% of the time and you didn't live in his head and know his thoughts. There is no way that these artificial memories are anywhere close to the actual thing. If they had done what the Scions did and anchored/copied their real memories into auracite (or a soul crystal?) while they were still alive, then perhaps their reincarnations could have gotten their real memories back later.Htyhlodaeus in Etched in the Stars
As you will have gathered, each crystal bears an account of the life of a member of the Convocation as remembered by the unsundered.
Perhaps this is a source of the resentment of the sundered towards the unsundered; Elidibus and Emet kept telling them that they were best buds and this is what they did together... but the unsundered know that wasn't their real memories. All the sundered Ascians know is that they were minding their own business living a normal life when suddenly, one day, these two dudes showed up, suddenly thrust these "memories" upon them that might not even be real. "Were totally your best friends! You need to help up blow up your world and kill your family and everyone you knew, to restore these people you never even knew about until we told you about them, who you care about because we say you care about them! And don't think about NOT helping us kill your family, because we're going to watch over your shoulder 24/7!". I wouldn't be surprised if - after Emet and Elidibus were suspected to be dead - that some Ascians decided to waltz off and go be with their wives and kids they had prior to being kidnapped and "uplifted".