I was going to say that it was a very small sample size to be basing statistical judgements on, but then SRH59 came in with exactly the same ratio. My conclusion is that Smashwords make them up.
Smashwords is unusual in that a lot of its business is with unknown writers. At a guess, I'd say the customers will be more willing to download a sample and also more willing not to convert if it's not what they're looking for.
What I mean is that, on my Kindle, if I grab a sample it is because I am already considering buying the book. If I was deliberately seeking new material, as on Smashwords, then I'd need to sample more to find something.