It looks like the cab had the typical hand wear along the side edge. The owner (or somebody) thought it looked bad and had these options:
1) Restencil the cab - no stencils currently available
2) Buy expensive vinyl replacement - nope.
3) Painstakingly hand paint the bad parts - nope.
4) Tape off the bad area and paint it black - yep! Whatever's quick, easy, and cheap. Probably did it to make it look "better"....