Steve Guttenburg of CNet explained:

“The impedance of a headphone is largely determined by the driver’s voice coil, and for Beyerdynamic’s high-impedance models the voice coil’s wire is super-thin, just 0.018mm, half the thickness of a human hair. Beyerdynamic’s Senior Product Manager Gunter Weidemann explained…

The thinner wires have more windings (layers of wire) on the voice-coil than the lower-impedance Beyerdynamic headphones, which have thicker and heavier, easier-to-manufacture voice coils. The lower moving mass of the 250- and 600-ohm headphones’ voice coils is lighter than the 32-ohm models, and the lower mass is part of the reason high-impedance headphones sound better. The smaller diameter of the 600-ohm voice coil wires allows the wires to fit tighter, so there’s less air between the windings, and that makes the electromagnetic field of the voice coil stronger. All of that reduces distortion for the high-impedance versions compared with the low-impedance headphones.”