Did some research. It's absolute, not relative
https://eur-lex.europa.eu/LexUriServ...46:0058:en:PDF
So, to answer my own question, yes my 225/40 tyre with A rating has lower cooefficient of friction than a skinny bmw bridgestone that "only" has a B rating.
BMW i expect though did the maths and decided that best option for them was compromise of narrow for aero, but grippy compound for safety/putting the electric torque down even if it meant marginally higher rolling resistance.