At high load (very low vacuum), the vac advance drops out. So, under load when you floor it, it goes away and you're left with the initial + mechanical, limiting total advance to 30ish degrees to avoid pinging.

At idle and cruising, you should have 10"+ of vacuum, so the vac advance is adding to the initial + mechanical, just like ported does. Giving you 45ish total, cleaning up the burn and improving MPG.

Ported just has zero vacuum at idle, but otherwise works the same when you're above idle.