Don't recall if there are WNPC's in Nimbasa, but in 9.96.5 there is a known bug that causes the starting frames for Wondercards to be slightly off in the main window (to quote the development notes, "Wondercards frames are incorrect, they don’t match between main and time finder (main is correct) PIDRNG starting frames might also be 1 off for them.") That's probably what you were thinking of, as you said.I just RNGed a shiny Regirock, and the starting frame was as it is in the Reporter (I.E. 14 advances.) Maybe I thinked that because I recall when I RNGed a Genesect and Keldeo Wondercard, the starting frame was a bit off in Nimbasa City PokeCenter.
Kadabrium was trying to account for the +2 difference (see this post for more info if you're interested)