I think you are right and the difference in the luminance of the 12 o'clock markers on the photos shown is due to older dials having a mixture of tritium and superluminova (tritium only for the 12 marker), with the tritium fading over time. One can tell the difference because the faded tritium turns yellow over time whereas the superluminova remains white.
Personally, I prefer the all white superluminova only newer dials, but the not-glowing-in-the-dark-anymore tritium markers at 12 have their own charm, both are correct, the latter is older.
D.
Cheers, Dirk