We're finally at the part of the story in which Grace gradually changes based on the input of Sarah and Ellen! It only took... This is the 51st comic in the storyline? That's not important no one dwell on that why did you even bring it up? Honestly.
When I first thought of Grace having some sort of memory advantage, I had considered it something Grace would have because "alien hybrid, DUH". This was also the "explanation" for her being good at math and foreign languages.
I feel a lot of these things retroactively make more sense when one considers evolution and uryuoms. Math is a more natural thing for Uryuoms for reasons to do with their shapeshifting, and they evolved with minds that are basically advanced calculators relative to human minds, ready to absorb and process a large amount of information quickly, though with little need to keep most of that information in the long term.
As for language, part of their shapeshifting for survival involved avoiding conflict, and assuming forms that would allow them to get along better with potential enemies. Part of this involved quick adaptation to languages other than their own.
Is that so perfect an explanation that no evolution expert could possibly glare at me because of it? Possibly not, but heck, by EGS standards, this is downright hard sci-fi, and I like considering how species would work and behave based on their history as a species and what would have mattered for survival.