I think, basically, it being a game where you build stuff for the sake of building stuff, a bunch of people will have bought it out of curiosity, spent half an hour or an hour playing it, got disillusioned by the lack of a 'point' and returned it.
Of course, the lack of a 'point' as such is absolutely intentional and I'd presume the people who do keep paying it like it *because* of that rather than *in spite* of it.
Thinking about it that way (which I did have to work through in my head first, this is Not My Area Of Expertise at all), the numbers don't seem particularly surprising, and I'd guess the article didn't comment on it because the people involved in the article knew what they were talking about sufficiently that they didn't find it surprising either.
... that or steam return rates are just higher than I thought in general; it would be difficult to overestimate my level of ignorance here.