I think this is as Andy said, the result of the old 'hand' lumed dials. Some lumers (new word), may have been more heavy handed than others. This is also why, I believe, you end up with the I-Beam style dials as well, as they were too afraid to lume outside the lines.
Add to that, I would not be surprised if, back in the day, when serviced the lume would get added on top of some of the older lume.
Both my Gen 1680 (for Tritium) service dial, and Ubi's 1665 (for Tritium) service dial, have the white printed markers, but no actual Tritum applied. That was left to the servicing tech but then the radioactive scare hit. It is not inconceivable to find Gen Rolex's with Service Dials and just painted markers sans-lume.... But I digress.