Interesting. What diseases are you referring that have this one to one correlation assuming, of course, there are no errors in the 23andme test results?
Off the top of my head - several varieties of Muscular Dystrophy, hemochromatosis, mitochondrial diseases, diseases caused by genetically-induced vitamin transportation/use/etc (B12, D, biotin, folate), diseases caused by genetically-induced enzyme deficiencies (hypotonia, lactic acidosis), etc.
I think many intelligent people get confused due to the way which Yasko and some others (mis)use SNP data. They take a lot of SNPs with very minor, non-disease-inducing effects, then fail to indicate how little effect they have, and/or badly sensationalize the effect which those SNPs have.
Many SNPs have an effect, but only some (tens of thousands?) are known to have a strong pathogenic effect. But for those pathogenic SNPs, it's very much the case of flipping the disease switch from "off" to "on". Whereas the other SNPs with a minor impact are more like turning a dial a couple notches, maybe from 0% to 2% on a dial that can go to 100%. And then other genetic or environmental or infectious factors have to turn the dial the rest of the way to get to a disease state.
It gets even more confusing because things like the Yasko panel combine the SNPs with tiny effects with SNPs with major (turning the dial to 65%) and/or pathological effects, plus a few completely harmless SNPs. But there is an abundance of real science showing quite clearly that certain SNPs (and certain pairs of SNPs on opposite strands of a gene) are 100% disease causing.