500 kcmil is .519 sq/in, two 4/0 is .520 sq/in. (per table 8)
If you used 310.16 you get 380a in the 75c column for the 500 kcmil and the derated ampacity for paralleled 4/0s is 416a (80% of the 90c column 520a).
Evidently Canada sees this differently. Maybe they share Gerald's disdain for 310.16
