Every time I've ordered something from Amazon, they always push Prime for fastest shipping. Then when you turn that down, they give you an option to pay a little more this one time to get your order shipped faster. I always pick the cheapest shipping option. The order generally comes just as fast as the fastest shipping, without paying the extra money.
They are going to ship according to whatever is easiest and cheapest for themselves if you pick the cheap shipping option. By now in most large population centers, Amazon warehouses are already stocked. Until they start cutting shipping costs, it will almost always arrive well before their estimates. So just save yourself the money and ditch Prime.
From what I've seen, the difference is that they either ship it same day, or wait a few days to ship it. It still arrives in the same amount of time from when it ships until I receive it, they just take their time (maybe artificially) in getting it out the door.
Exactly this. Once you place the order Amazon still has to get it from the wearhouse and ship it too you. If someone is already picking orders in that part of the wearhouse it makes sense to just grab it now rather than wait a few more days. Same with shipping. If the order is together they aren't set up to just hold it, they ship it.