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...Ultimecia: Your feelings, I shall summon the most powerful of things
[from them]! The more strongly you feel, that will be what shall torment
you. Fufu.
...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.Originally Posted by Scan data for Griever
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.