My guess--and it's only a guess--is that at one time they were all part of one rooming house. I know that's the case with other properties a few blocks away (in the direction of Massachusetts Avenue). And, most likely, that occurred shortly before or during World War II. There was a huge housing shortage in D.C. in the early 1940s and many properties in that general area were changed (or already had been changed) from single-family homes to rooming houses.
Here are some views of those rooming houses during World War II: http://www.estherbubley.com/owi_boarding_house_frame_set.htm
Hope that helps.