Yeah, that really bugged me about the Okuda Chronology. Like how they put the Valiant's flight to the edge of the galaxy in 2065, just a few years after the first prototype warp flight, in order to put it exactly 200 years before the second pilot, rather than assuming it was rounded up from 170 years or something. Or the way they stretched out the third, fourth, and fifth movies to be a year apart, in direct contradiction to the internal evidence, apparently in order to justify Nimbus III being exactly 20 years old in TFF rather than rounding it down to 18 or thereabouts. (And yet they put TWOK 18 years after "Space Seed" instead of the explicitly stated 15.)
To be fair, they were trying not to speculate too much beyond what was stated onscreen, so if somebody said 200 years onscreen, that's where they put it. The Okudas would be the first ones to consider it a placeholder date to be contradicted onscreen at the whim of the episode writers.
Regarding the timeframe chosen for the films, I was always under the impression that they were trying to stay true to the offscreen sources that specified the first season of TNG as taking place 78 years after the time of Kirk's crew. 2364 (TNG Season 1, the first current Earth calendar date ever specified in Trek) - 78 = 2286, the year that the most recent installment of the TOS movie series took place (STIV). Putting TWOK and TSFS a year earlier wasn't much of a stretch, as the three films spanned over three months and there might have been a New Year in there. Putting STV in 2287 on top of that was a stretch, though.
What was too cutesy was putting TOS exactly 300 years after the TV seasons aired. They should have taken a hint from TNG, which didn't take place exactly 400 years in the future of its broadcast. Nudge the TOS dates up by two or three years and there's no discrepancy with the 15 years specified in TWOK.