As you say, the same appears on many of his other shields - often with the charge actually overlapping the line. My best guess is that it's meant to represent a physical shield with the centre part being slightly raised - though that is only a guess, with nothing really to back it up - and the fact that he doesn't do it on all his shields is somewhat counter-indicative as well.Moesia also appears to show a bordure like division but the hatching is the same either side
Perhaps a tressure?
Or perhaps to indicate it's an escutcheon on a shield of the same colour and any outline will do. Though that seems odd to me.
Edit: or maybe not? Looking at those other shields linked he's done something similar. Weird!