Rounding off, up or down large numbers is not a problem when they are given as a general description. Hence, "2 centuries" or even "20 years ago" could easily be a few above or below those figures.
However, more precise numbers given by characters with a definite point to prove are less easy to ignore - why would Khan say "15 years" if he could say "16" or "18" or "20"?
With the TMP situation, Decker clearly says "over three hundred years ago" so it really can't be any less than that. Unless, as someone upthread has postulated, he was just really lousy with history? This is the "old world calendar" we are talking about, not a Stardate to be seen!
Maybe Voyager 6 fell into a black hole after
being swept along by one of those magnetic storms that captured the S.S. Valiant?