See, in d20, you would have like an angel or dragon or something with Int 21. That would turn into FC Int 15, rounding down to 14. But look at the fire dragon with its Int of 14; in AD&D a red dragon rates as Exceptional, while in d20 it would have an Int of 14 to 18 in the Huge size category. Even though using the AD&D intelligence score might result in slightly higher scores in some areas, I think it's closer. Anything over 18 represents an unnaturally high ability.
Particularly, you convert "Godlike" intelligence to Int 15. Godlike?