To clarify further -- Appears AC AP, as does UA, will "always" offer in their program on their metal, award tickets. UA use to call these Standard awards, and generally are pricey (use to be atleast 2x saver awards, now can be even more). The award space offered to partners, X for economy, I for business/First, is much more limited but cheaper. So if checking for award seats on AC, using AC miles, there may more award space for that AC flight then you will see booking on UA.com, using UA miles. This differential, is common across *A partners, more award space for their program members than for partner programs. So UA offers Standard Awards and XN, saver award space for UA miles only. Partners have access only to X award space. AC AP likely does similar things.