While I agree w/JCL in theory, and from seeing my range often climb after a fill, then out on highway a while later. But, I have never seen that large of a number on our E53...500-550, yes; never in the 600s.
Our Vette will show low 600s, after a fill to the brim, and then easing out onto the interstate, and rolling up to cruise speed, after a few miles.
Our X has done 550miles+ on a single tank before fill, but I have never seen '600+ range' show up. Dunno what your number means...a temp glitch, an anomaly, the EPA guessers at work, a politician's promise? ...
GL, mD