I was similarly amazed by this phenomenon. Doesn't make much sense on paper, and the real world difference (speaking of straight line speed only) is likely negligible. I believe the M5 could probably break the 4.0 second mark, but I don't think the GC could do the same.
I guess what I'm saying is the M5's time may be more conservative while the 650xi GC is closer to reality.
My 2012 650xi coupe is reported at 4.7, but I think it's really closer to 4.5. Apply this logic to the 2013, and the 650xi coupe and M6 are nearly the same - but, again, the BMW time for the M6 is likely on the conservative side.
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