I don't think that's the reason, chances are this is a 1070/1080 reference board design with the minimal changes necessary in the printed circuit layout to accommodate the 1060 chip. GDDR5X has more pins and a lower voltage than non-X and gains most of the performance by allowing a quad-data rate mode with larger prefetch. Essentially it's a more efficient but more expensive(Wider) design, so presumably these chips were going to be written off otherwise. There's a chance the memory controller on some of the 1060s weaker chips is a limiting factor to boosting speeds beyond original spec, though these chips should have a small power reduction & possibly better OCing potential.