I can't answer for everyone, but I'm guessing in many cases it has to do with cost of living.
Our income is almost 2X the median for our city and at least 2.5X the median for the state, but we can't even afford a decent SFR in our area, at least not using any traditionally accepted, sound, financial metric.
Why? We moved to a location that experienced a huge house price bubble, not to mention the effects of prop 13, which protects a specific demographic, but punishes any and all future home owners. I know, I know, blasphemy in the state of California, but it's true.
I recently pulled sales numbers and tax records on 2 identical tract houses in our neighborhood (where we rent because we can't afford to buy)
House 1
Sold for $320,000 in 1998
Yearly property tax $7,634
House 2
Sold for $1,040,500 in 2008
Yearly property tax $15,322
Prices are down from that 2008 number now, but not much. So how does that even make sense? A neighborhood that was originally targeted to, and affordable for, middle income households now requires an income solidly in the top 5th percentile, if not higher, for the numbers to make sense.
Yet every politician out there is pandering to the masses about putting a floor under housing prices. Umm, why? More money would be pumped into the economy if housing was actually affordable given actual incomes.
Anyway, very long post just to say that while I think every chart quoted in this thread puts us solidly upper middle if not higher, we certainly don't feel like it, and we wouldn't classify ourselves as such... especially since we're just renters.