Different outlets (and different Saks in general) have different stuff. I don't think the SF Saks carries Isaia, so the Off probably got old stock from the Vegas store or elsewhere.
Oh boy! I am awaiting on a $2,000 freaking...Warning: Spoiler! (Click to show)
My Internal Reaction (Click to show)
^Saks never carried Isaia. NM was the only place where I saw them, along with TF.