I'm actually surprised it's not a LOT higher, considering they are talking about overall net worth, including mortgage.
I actually think it's amazing it's only 1 in 7....
Well, a mortgage is backed by an asset, so unless you are under water, you get a real push in the positive direction.
Maybe I'm reading it wrong, but the way I interpreted it was that if you have a 250k mortgage on a house you 'paid' 300k on, you'd be -200k in the hole.
I think you're doing the math a little funny. For net worth, you'd consider the total value of the home (i.e. $300k) as an asset, then consider the mortgage balance ($250k) as a liability. So your example would end up +$50k for house&mortgage contribution to net worth.
Mortgage lenders are a lot tighter now on LTV, so it's unlikely to see someone underwater on a newer mortgage (FHA loans with just 3% down and/or property value drops could still cause it, but it's much rarer). But there are also still many old mortgages on the books originated back when home values were much higher in some parts of the country.