Walmart 2-day shipping is a lie. There. That is the difference. When you order from Amazon with a two-day delivery, you can reasonably expect Amazon will hit that goal, pretty much all the time.
I haven't ordered anything from Walmart, so I can't disagree. But when the last 5 out of 5 Amazon Prime shipments arrived late, that was the end of my Prime membership. I usually buy things off of Ebay, and the last Prime items I bought from Amazon were all available from Ebay significantly cheaper, I paid more because I actually needed them quickly. Now I just assume all fast shipping times are a lie.