Via Yglesias: Austin Contrarian: Density calculations for U.S. urbanized areas, weighted by census tract
The Census definitions are weighted by area, this is weighted by census track population, and produces a different, and some would say more intuitive result.
An interesting calculation, and goes back to the same issue regarding Observation Bias that comes up with transit ridership, although in this case, the observation bias may be weighting by area rather than population (so long as everything is properly defined).