I do not know why this is, but it would appear that a number of 'reliable' companies are sending out items for next day delivery at no extra cost to the customer. Amazon, Tesco Outlet, Tool Station are but three companies of many that do this, and who I use on a regular basis.
No doubt, you will get the odd complainer saying the item was delivered, when they had not expected it?.
To me personally, I am all for good customer service. At least it proves that some companies will go that extra mile for their customers, and perhaps not regard their customers as an inconvenience!.