Hey Marcarl,
I have to admit, I have rarely seen you post about something that has you scratching your head over it, so this will be an honor to add my two cents to the mix! With that being said, The guide bushings that keep the upper fork tubes in the center of the lower fork tubes will wear out just like the fork seals. In fact, if the guide bushings are excessively worn, they will cause a perfectly good set of fork seals to start leaking!
Because of the "Rake" of the front fork tubes, (angle of the tubes in relation to the center line of the motorcycle frame) the guide bushing inside diameter will become "egg shaped" over time. When this condition increases to the point where the lip of the fork seal no longer contacts the upper fork tubes, leakage will result. There are usually two guide bushings in each lower fork tube, try replacing those.
Earl