How many Kindle ereaders were ever sold at Waterstones in the first place? A fairly low number I'd imagine since most people will buy direct from Amazon, or from places like Tesco & Argos where they quite often ran deals. Waterstones would have been the last place I'd have looked to buy a Kindle from.
Like Waylander says, most people go to Waterstones to buy physical books.
Seems a bit like a garage that also stocks cycling clothes coming out with a press release saying, looks cars are really still popular we've hardly sold any cycling gear whereas loads of people still come to us for car parts?