That's strange, Take Newegg (both instock):-

WD My Book 10TB USB 3.0 External - $185.99 (free shipping) https://www.newegg.com/p/N82E16822234346

WD Red 10TB NAS Hard Disk Drive $283.98 (with $3.99 shipping) https://www.newegg.com/p/N82E16822231546

So it's still ~$100 cheaper. Like I said before, this isn't the only example.