House depreciation in Seattle

I came across this odd map posted by Zillow:

Seattle-Tacoma-Bellevue, WA MSA - Map of Real Estate Market Year over Year Home Price Appreciation

The areas are not that interesting, besides saying that Bellevue got hit with over 13% depreciation, which is pretty big (and my house is in the tip of the area that only was down 0-11%). What I found interesting is the ranges:

<0% (appreciation)

Note the 2 percentage point range of the blue region. Seems too arbitrary... Fishy...