I feel like these Ship Manager options are cursed so I'm asking here and not elsewhere.

I noticed picking "FedEx Box" is sometimes cheaper than keying in the same box dimensions by hand. Why exactly does this happen? I heard somewhere it's an incentive to help improve sorting. Do those boxes get preferential DIM factors or is it caused by rounding?

For my example I'll ship a 1-2LB FedEx® Extra Large Box (X1: Cube-shaped) across the entire US. You probably wouldn't use such a big box for 2LB but it makes it obvious. The dimensions are 11-7/8'' x 10-13/16'' x 11-1/16'' (30.16 cm x 27.46 cm x 28.10 cm).

2Day "FedEx Box": $16.29

2Day Rounding down to 11x10x11 (NOT allowed): $24.20

2Day Rounding up to 12x11x12 (correct way): $26.50

That's a $10 difference to ship exactly the same thing. It's something I check for now because it's occasionally cheaper to use a free FedEx box that's slightly bigger than to grab a box I own that's the right size.

Does FedEx even check if something is their own packaging? What would happen if you shipped a different box with similar dimensions and billed it as a FedEx Box? Or a Pak?

Also, why do Express Saver rates disappear if I select FedEx Box? I can get a rate for it if I use Your Packaging or FedEx Pak. It's like they don't consider Express Saver an express option.

P.S Is there any tangible benefit to checking "This is a residential address" on Express shipments since they don't get weekend delivery anyways? I notice if you miss it the residential surcharge is tacked onto your invoice with no penalty rates.