Similarly I'll get something I order in a day or two even when it is the 5-7 business day option - it's in their interest to make everything as efficient as possible and not be holding onto inventory.
Living near a warehouse or on a major shipping artery means things show up very quickly basically no matter what. The post office / shipping companies don't stall orders just for the heck of it. If it takes two days to go from here to there, even though you paid for slow shipping, you'll get it in two days.
Sometimes I wonder. I've definitely noticed packages sitting in the local post office for a day or two to simply get past that 2 day limit. There's even been times where I was able to call the post office and go pick it up, because they didn't send it out for delivery despite arriving the day before. Could be innocent, but it always struck me as suspicious.
I think the main benefit is your orders get higher priority during holiday season when it's busy and regular orders may take a long time to fulfill.
You're likely right it does put you in a higher tier of priority, however how that plays out practically be a relatively slim advantage or speed increase - as is my and OP's comments allude to. Comes back to the rule of efficiency and lower cost with adequate planning, planning ahead of time, detriment of impulse or last minute buying, and stability of supply-demand systems.