Haha, it’s a bit funny that you ask because the numbers actually derive from your default x3/x4 settings from the beginning! So I would have asked the same of you, originally, but then I started sort of “reverse engineer” it to try and figure it out for myself… 🙂
Now, just a disclaimer I am not an engineer or such and I just recently got interested in this analog video signal stuff because of the OSSC. Wanting to inform myself, and possibly help others, I did quite some research trying to figure out why certain sample rates worked on my displays, and this is how it makes sense to me… But maybe my explanation is not entirely correct.
My conclusion was that the reason behind 390 should be pixel aspect ratio (PAR) correction, to achieve correct display aspect ratio (DAR) when mapped to the 1:1 PAR fixed pixels of modern displays (with 480p as a notable exception). I got my info on PAR from this page.
PAR for 256 active width dot clocks (5.37 MHz, SNES etc) is 8:7, and 341*(8/7)= 389.7.
PAR for 320 active width dot clocks (6.71 MHz, Genesis & PSX) is 32:35, and 427*(32/35)= 390.4.
(And btw the actual aspect ratio of graphical content is not 4:3 but ~1.306, but I think I’ll leave that for another post…)