That just sounds too logical to be plausible. I'm undecided between NORG's brunch or hidden traps.
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FUSHUFURU
The Grendels came from the island closest to hell. There NORG was leveling up and captured a few of them. When the Grendels came with NORG back to the garden the monsters from all around joined in to fight on the day squall and friends came back from the desert prison.
NORG is what almost ruined the story for you? That's it?
Sorry, couldn't resist. Ignoring criticisms of FFVIII's plot, I never really paid so much attention to the fact that non-training center monsters were present to really care. It didn't exactly seem like an issue to me. I think there are far better things to consider when asking, "What the hell were the writers thinking?!"