You’re seeing how changes in circumstances cause changes in estimates. If the first time the next truck going out was tomorrow because today’s was full, but now they have a promised overnight delivery for a bunch of stuff going in the same direction, they might put your package on that plane instead. If you have Amazon Prime then your membership pays for the extra fees for the faster delivery that is now available, but otherwise you’re probably going to pay more if you select that faster option.

Or maybe there was a return or cancelation for that same product and it happened to go to a warehouse local to you, so now there’s a closer option.

Lots of things change and Amazon tracks that stuff in near realtime.