When trying to figure out actual Prime shipping item arrival times these days you have to look at each listing and see what the guarantee is. Depending on the time of day and actual stock available items will either have “Want it by date? Order with in XX amount of time and choose two-day shipping.” Listings that says that guarantee will actually arrive in by that date. If there is no guarantee sentence and it just says “Free Two-Day Shipping” or “Prime 4-5 Days” then the item will not actually arrive in two days. Free two day shipping items mean the item is out of stock and will ship via a 2 day shipping option once Amazon gets it in stock and is available to ship it. Prime 4-5 days is when an item is in stock however takes an extended time to fulfill (Small & Light program items usually have this banner which is why you are seeing it more commonly as the program becomes more popular).
Amazon is trying to offer more items under the prime badge while still saving money on parcel shipping and not actually shipping everything for delivery within 2 days of your order.