I'm not an Amazon Prime customer (shocking, I know), and I noticed that the shown delivery times are overly pessimistic, probably in order to make you opt for a Prime membership to get faster delivery. And then the package arrives at least one day earlier than the predicted delivery date.
I gave up and canceled my Prime subscription years ago after repeated failures to meet even a 2-day delivery promise. I never once got anything same-day. And the Video and other services of Prime were things I never used.
I'm not a Prime customer. I'd wager close to 50% of my orders arrive within 2 days. I'm glad Prime members are subsidizing this :-)
Even more shocking: I'm not an Amazon customer at all. Which, granted, is probably easier if you're outside the US.
FWIW Prime also often delivers earlier than the set date (yes, even when it's set to next day...)
I assume they want to limit the amount of customer support more than advertising their logistics.