Advance multiplier doesn't actually affect the fuel mapping. That comes strictly from O2 sensor feedback. Less air teaches the car to run less fuel under any given load cell in the map. Drive down the hill and you outrun the O2 sensor learning, effectively leaning out the car a little bit.
Advance multiplier is a fraction that determines how much timing advance from the "learned" table gets added to the "base" table. So in cell X of the base map you might have 12 degrees of advance, and the corresponding cell in the learned map might have 8 more. If the car isn't knocking and the multiplier gets all the way up to 16, you'll get all 8 degrees of advance from that cell. If the car thinks the fuel is a bit lower octane because of knock sensor feedback, it reduces the multiplier, and you get that number divided by 16 times the amount of advance in the learning map. Short answer:
multiplier=16; 16/16*8= 8 degrees added advance
multiplier=10; 10/16*8= 5 degrees advance.
The ECU will always be looking to nudge the multiplier up because the characteristics of the EJ20 make timing advance the more important factor for power than actual boost. (IOW, it's better to add timing without knock than to add boost and pull a little timing. Hence the "MBS are bad" stance of the Subaru tuning community at large.) And in DZ's case, it wants to kick the multiplier kicking and screaming up all the time.
