The seven-fold circularized propellar is called a b propellar by the authors.
From the name, it is clear that seven b sheets are needed to give the b subunit of the G protein the correct shape for its function.
You should note that the terms
"b sheet" and "b subunit"
in their names only by coincidence.
The subunit that carries the propellar could have been called the "a subunit" but by chance it was called the "b subunit".