So they looked no further than production date, and added three hundred, fair enough makes sense from their perspective despite some anomalies.
Was interested in this point though
The age of Sarek in "Sarek" had been set at 202 by following this very doctrine: the dating of that TNG episode was carved in stone already (it was 2366 because it was the third season and the first season had already been mentioned as having taken place in 2364) and going 100+300 years back would bring us right to the airdate of "Journey to Babel" where Sarek was 102 years old. Had the writers not been following this doctrine already, they would have been quite unlikely to choose 202 as Sarek's age...
According to the official chronology Sarek occurs in 2366, fine but they place Journey to Babel in 2267 not 2266. a gap of 99 years.