They're all Northeastern states imo.
Being on I-95, Acela, safe Dem since 1992, colleges generally being in Northeastern conferences, Baltimore and Washington not really following the development arc of Southern cities, Delaware pretty much just being a suburb of Philadelphia, density of a Northeastern state, they weren't part of the Confederacy, not really part of the Solid South, etc.
The lack of forests in DE/MD (east of Hagerstown anyway) makes them Southern. Also the wide open fields. There are so many variables to choose from.
Wait, the south lacks forests?
Yeah, all of these trees outside my house are leftover props from the local community theatre set up as such to confuse northerners.