Applying the calculation after a simulation caters their contrast ratios to that impairment. Perceived luminance differs if you perceive colours differently. You're right that the ratio doesn't change with a specific impairment (and it's not), it's just applied to a new set of perceived colours.

WCAG contrast ratio is purely based on brightness contrast (not even accurately perceived luminance). The requirements does it's best at creating a baseline to cover minimum accessibility, but it's still based on one version of contrast perception.

Ayy didn't expect this to pop up haha, thanks for sharing it mate 🙌 working on a bit of an upgrade for it atm, so let me know if there's anything you'd like to see in the v2