Chromecast Amazon Sales Charts

So heres a mystery maybe you guys know the answer for. So in amazons Best Selling Electronics a Roku is #3 and the Chromecast isn't anywhere to be found. BUt in Computers and Accessories, Home Audio and Theater, and Television and Video Products (all subcategories of Electronics where I saw it or the Roku) it's beating the Roku out and infact ranked as the #1 best seller. Any idea why the numbers don't seem to agree? Are Amazons sales charts just completely useless and unreliable? Or are they only reliable for top level departments?