Next Day Delivery does NOT mean you are going to order an item and get it the next day. Usually with Next Day Delivery, you get it 3-5 days later (I have ordered atleast 100 items online, Im a pro at this!)
Next Day Delivery simply means, that the day after it leaves the post office it will arrive at your house.
I have had Next Day items, that took two weeks to get here.
If you have a FAST shipper (one that runs out to the post office with each order), and you purchased it fairly early in the day (noonish), and there are no weekends within the next day or so, then you could possibly get it the next day.
However, if your shipper isnt super fast, and gets to the post office late (or even waits 24 hours like most do) or you order the item late so even if the shipper ran to the post office they wouldnt get there in time for "todays" shipments, or if there are weekends involved, it can take 3-5 days total.