Have you heard of a floating time line? It's like how the Simpsons keep moving Homer and Marge's marriage forward so they'll alway be married xx years ago.

That's basically the reality for Ultimecia, and Orphan who live in the future but not a particular time. They're an inevitable whenever.
Ultimecia wanted to compress time to 1 memory. Because she's like a Simpson's fan that sees the floating timeline as an absurd thing, when characters like Skinner suffer ptsd, from a war they couldn't have been in.