Smart spam?

Occasionally I'll get comments that are clearly spam (since this blog is rarely updated, feel free to replace "occasionally" with "rarely"). Ordinarily, I delete them without a second thought. Two recent comments, however, gave me reason for pause.

I am talking about this and this. These comments are a bit of a mystery to me. First, note that they are not only syntactically but also semantically more or less well-formed. They are even vaguely relevant to the post's content! So a natural hypothesis might be that they were written by humans. But then why the obviously spammy aliae (aliases) --- "buy sildenafil citrate" and "generic viagra"?

Explanations welcome...