Key observations:
* All else being equal, we would expect 50% of homes to be unaffordable to a median buyer. This is the reasonable base figure (not zero). We would expect 2 in 5 if everyone were buying homes they could properly afford.
* But they're comparing median county incomes to homes in a metro area. For example they call out Boise, ID but use income from Ada county (already 20% lower!). This issue alone could account for the difference.
* There is also an issue of counting homes for sale, vs homes in general. If more expensive homes are sold more often (or, are listed for sale longer - say years - because they're unique) then we will see a skew towards more expensive homes being listed more frequently. It won't accurately represent the actual underlying home inventory.
I recommend not reading too much into these numbers.