Quote Originally Posted by Masamune∙1600
That's because the idea (that Griever was Drawn from Squall's mind) has evidence to support it.

Ultimecia obtained Griever by Drawing the parameters for the being directly from Squall's thoughts. The direct Japanese dialogue drives this home with heightened clarity.



In the context of the precise Japanese dialogue, Griever's scan data...



...can be interpreted to support the supposition that Griever was "created on the spot." Griever, in Squall's mind, was the ultimate GF, and, using those thoughts as a template, Ultimecia brought forth a GF to battle the party. This view makes even more sense in light of the fact that Ultimecia can blow away stocked spells, suggestive of her ability to affect and otherwise utilize memory.

In any case, as suggested by the FAQ, the Ultimania Guide for FFVIII states that Sorceresses do not have extended lifespans. Since Ultimecia lives many generations in the future, there's no logical way that Rinoa could be an active presence at that time (generations in the future). Therefore, one should conclude that Rinoa and Ultimecia are entirely different individuals.
Both theories have info to support it, that's why we don't know which one is true 100%. FF8 leaves alot of things questionable.