New York has this... I don't think it's atypical. Tracts are so small that it only takes a single development to raise or depress median income grossly. Take a look at the middle of Manhattan: there are tracts with $10K next to tracts with $120K, but they might have as few as 20 households in tract, which means that a single building with 11 low-income tenants would pull the median down to the poverty line.

http://www.wnyc.org/blogs/wnyc-news-blog/2011/dec/08/census-...