DC to Baltimore in a straight line is 35 miles, Los Angeles to Anaheim 25, Dallas to Fort Worth 33, Seattle to Tacoma 25, San Francisco to San Jose 43. A lot can be said for DC and Baltimore forming one polycentric urban area. There's no distinguishing the continuous development midway as being part of the urban area of one over the other.
Chicago to Milwaukee is not really the same. When you're going to Milwaukee from Chicago, there is a clear (and considerable) area where the Chicago metro area has stopped. It's sparsely-populated farmland. You don't get that with Baltimore to DC. It's built up and populous all the way from one to the other.
Should have known Houston was on the water because of their NASA connection. Washington and Baltimore are too close together to sort out on a map this size.