But lots of C++ books. C++ and Python are my primary languages, and I own far more C++ books than Python. For C++ I've got all Scott Meyers stuff, Alexandrescu, Stroustrups C++ language. For Python I've got Beazley, and that's it. My guess is that Python, and probably Ruby too, are so much easier to learn than C++, so fewer books are read and sold.
I don't grant the premise that fewer Ruby and Python books are sold... Regardless, the number sold is greater than 1 so they should really be listed in the app.
I'm pretty sure it skews old by not correcting for publication age. That's how "heads up design patterns" from 2004 beats almost everything since, simply by being older. In 2030 something newer will have higher numbers, but an old classic will have even larger numbers, so ...
Eventually old stuff tails off. I learned C from K+R edition one in the 80s, and the ANSI edition two has now displaced it.