If the Rod you're using is Mercury-Pure, would it ignore a Salt-pure mine of comparable purity value?
If this is the case, then that means that when looking for high-purity mines, not only are you looking for rarer deposits, but you're ignoring 3/4 of the ones that exist. If not, then you're merely looking for things which are statistically rarer.
I suspect that it's element-based, but I do not know. All I know is that my woo-woo wand has found 8 different mines, and my 10/10/40/40 Dowsing Rod hasn't found anything.