Not sure where you got 7.5mm from?? 2.5mm flex is what they are. It also comes down to strand count. High End speaker cable has a Shit load more strands than standard flex. Which is more surface area inside the cable for current to travel down. There is a point where it gets ridiculous with buying cables I would sit in the middle somewhere.
3cores/cable x 2.45mm^2/core = 7.5/cable.
The norflex are 14 solid cores that add to 1.65mm^2/cable, compared with a few hundred for flex..
Still, even if the strand counts were reversed any skin effect would be swamped by the difference in cross section.