Important difference: all Roots blowers move air in the space between the rotor lobes and the case wall, nothing passes between the rotors (which almost completely interlock).
A twin screw type compresses air between the rotor lobes. The lobes never completely fill the opposite empty (mirror image) spaces. The inter-lobe volume is greatest at the intake port and gradually shrinks (lobes fill more of the space) until released in the manifold.

w/r/t screwed to the rear
All helixed rotors move the air forward, look how small the Eaton discharge port is (compared to the full length G.M.C. etc. window).
"Better" was invented 27 years ago, and no one caught on...