How is Fast GI calculated in the combined pass for Cycles?

Apologies if this is not the correct place.

I have searched widely and asked on different community forums, but sadly no answer yet. Hoping to reach some Cycles wizards here.

I am trying to rebuild light passes into a combined pass. It works fine until I render with AO/GI. You can see the difference in results below:

Node setup

Combined Pass - AO off

Rebuild Composite - AO off


Combined Pass - AO on

Rebuild Composite - AO on

Since the manual doesn’t explain much about AO, I thought maybe this info could be added? Or at least a note or tip mentioning this.