This is just a guess, probably Microsoft expectation of replacement from "send to repair" system (where they pay ~500-550 U.S, from what I remember), and warranty replacement was higher then expected. As the SP4 was about to be released, they probably top SP3 production a while back. So now they need to kick-start it (well they probably done so a while back), the problem in production of computer devices, is that it isn't like a printer. You have to LARGE orders, you can't place 1 or 2. And, it takes months to produce, especially when you need to cater at a world wide market. Usually the order needs to be place 3-4 month in advance. So you start to see where predicting how much you need, is very difficult. Margin are small for computers. What I mean, is that if you compare a Surface Pro with a.. say phone case. Your phone case probably cost 0.10$ to produce (in a mass order), in a computer, every manufacture of parts (processors, chips, resistors, capacitors, etc.), wants to make a profit, and well, processors and high quality circuit board design, displays, and SSDs and all that, is not cheap either.
So you want to avoid a big write off.
So, usually a companies plays it safe. You get a new device, and not a scratched up device, but you wait, and Microsoft avoids or limits loosing a lot of money.