Apparently, β serves more as a v sound than a b sound. In the transliteration, the π is the labial plosive while the μ imparts voicing to it, from the labial initial position.
The title phrase is supposed to originally refer to British Prime Minister Robert Gascoyne-Cecil, after he appointed his nephew to an important post. Interestingly, a friend could legitimately say that “Bob was her uncle great-grandfather”.
But that’s how Greeks render the “b” sound of other European languages, just as they render “d” as “nt”.