What makes you think TMP was based two years after TOS?Thats the thing, TMP was based two years after TOS era, TWO years and all that difference between Enterprises, external and internal.
The actors were a decade older. There had been a lot of changes in rank. There had been a lot of changes in technology. Personalities had changed, too.
Nothing established the exact date of the movie; we only learned that it had been at least 2.5 years since Kirk had last been to space, and 18 months since the Enterprise had last sailed. Those things could have easily happened eight years after the end of TOS.
There was every possibility that the creators thought that TOS and TMP took place on their respective decades: TOS in the sixties (be it 2260s or perhaps 2560s) and TMP in the late seventies or even eighties. And having TMP in the late 2270s would be an excellent thing for chronology, as it would allow for Voyager VI to be launched "over three centuries ago" even if the first Voyagers were launched in the seventies...
"Though it was a blatant violation of the Prime Directive, Kirk saved the Pelosians from extinction, just as he had the Baezians and the Chenari many years earlier. Finally, in the year 2270, Kirk completed his historic five-year mission, and one of the greatest chapters in Starfleet history came to a close."
----- Icheb in VOY 'Q2.'
That places TMP in 2272 at the earliest and more likely in 2273.