Actually the date - well, the time gap - was deliberate. Since TMP tried to ignore the fact that the actors had aged a decade since TOS and tried to be only a two-year delay since, TWOK had to pad some years in between to explain why Kirk and company had AGED.
They said that it had been 15 years since they'd first encountered Khan because it had been 15 years in real life since that episode was filmed. Simple.
One of Harve Bennett's big things when he took over
Star Trek was that the actors should start acting their ages and stop trying to look like they hadn't aged since the series.
I'm pretty well on record at being slightly annoyed at how literally Mike and Denise took every line of dialog in Star Trek. It's not the only timeline issue that's caused by their doing that... Admittedly, they had a bit of a Herculean task before them, but a less literal approach to dialog would have made a lot of things much easier.
Well, I think that they didn't have too much choice, because:
A) In most instances it doesn't matter much if a particular incident was 49, 50, or 51 years ago (Which is why the conjectural date of Zefram Cochrane's first warp flight was adjusted from 2061 to 2063 between the first and second editions of the Chronology -
First Contact had moved the date two years forward in time).
B) Once you start interpreting
every date less than literally, where does it end? What's to stop you from saying, "Oh, well they
said 20 years, but let's make 30 because it fits better with our conjectural timeline?" If you're assembling a timeline out of something that's been written by dozens of writers over the course of 30+ years, you're going to have some discrepancies here and there. That's unavoidable. All you can do is try to do what makes the most sense with the majority of information. It's just that the "TWOK was 18 years after Space Seed" thing is one of the Okudas' more egregious misjudgments.
Hell, the Sherlock Holmes canon probably has as many timeline discrepancies in it as the
Star Trek canon does, and that's 60 stories written by
one guy over the course of several decades. Really, when you consider how that many people were making it up as they went along, it's a wonder that the ST timeline hangs together as well as it does.