Dear PoPville – Can Someone Explain DC’s Streets?

Photo by PoPville flickr user johnmcochran2012

Dear PoPville,

I live in NW on a east-west running letter street. While visiting a friend in NE yesterday, also on a east-west running letter street, I noticed that the street numbers of buildings are set up differently. My home in NW is an even numbered house on the south side of the street. My friend in NE lives in an odd numbered house, also on the south side of the street. Spot checking on Google Maps, it appears SE is the same as NE and SW the same as NW, so the switch is made at North/South Capitol St.

Does anyone know why this is or any purpose behind it?