its called understanding statistics and then applying them to the bigger pictures.
First, the article give the following
One. This is USA only. Not internationally. in 2013 only the following sales numbers:
AppleTV: 2m
Roku: 3.8
Chromecast: 3.8
However, the numbers you are using are international total unit sales over the course of the lifetime of the products.
Which has a variety of other factors into it besides just how many sold. For example, since 2007, Apple TV has been available in dozens of countries (Sure the content might not be great out of the US, but it's still available for sale). On the flip side, Roku is not quite so big or internationally known. Even here in Canada, Roku's weren't available through retailers till the last couple years at all (and usually a generation behind).