I think it's less fast shipping, and more reliable shipping. I ordered a pair of shoes from a major UK high street store earlier this year who advertised 3 day

I think it's less fast shipping, and more reliable shipping. I ordered a pair of shoes from a major UK high street store earlier this year who advertised 3 day shipping. Fine, I'm going away next weekend. Except, it turns out their guarantee is from dispatch, which is 3-5 days. It was listed in the small print on the order page, but even the order page still had the "Free 3 day shipping" banner on it. Unsurprisingly, they took 5 days to dispatch, and 3 days to deliver. I actually ended up going into the store to buy them, and returning them when I got home. Amazon, for all it's faults, if they say next day, it's almost certainly going to arrive next day.