Because comic books hold all the answers to questions on time and space?

But to be serious, it is possible that there was an original time that didn't start with Ultimecia arriving from the future, but which leads to her travelling back. This is a line of time which can be changed. After the loop is created, it would basically remain forever like it was, rendering it identical to the unchanging time as far as the loop is concerned.

Now, I believe time doesn't chage simply because Ellone says so, but there are reasons why she might not know the full truth. I'd link you to an FAQ I wrote on Gamefaqs on the subject, but I can't directly link there. It's under the FF8 section, called "Time/Ultimecia Plot FAQ".