It's a a PI (Public Improvement) plan. Makes sense in light of the sewer being built.
It's possible they will combine the properties, re-subdivide, build a cul de sac so they have the street frontage required to build more houses under R-1 zoning. It's also possible that the properties septic systems were failing and the only was they could re-develop was to add sewer. Call the engineers in charge of the project, they will know.....as though it's really any of you business....
http://ldsnet.fairfaxcounty.gov/ldsnet/PAWSMain.aspx?cde=PI&seq=4219937