Pitting from cell rot often is patchy - it often tends to follow the lighter and more transparent celluloid, whereas normal rust doesn't.
It progresses from the line of closing of the razor, yes, and in the example you show the scale celluloid is pretty uniform in material so I'd expect a more uniform pitting from the rot (although it's way too far gone to have any idea how it would have started). Look at my example again - the pitting is pretty much within the closed line of the blade, and it's almost an exact match for the light color patches of the scales.
There's another example of pattern-dependent rot
here.