An excellent question Rickers. Unfortunately, despite that impressive build up, without direct access to Yamaha's engineering staff all I can provide is a semi-educated guess. Valve inspection intervals are dependent on many factors, including cam design; radical cams tend to be hard on the valves so they require more frequent adjustment. The quality of material used for the valves and valve seats matters too. Things like the valve's size and weight, the type of adjusters chosen and even the compression ratio of the engine all play some role in the process. Furthermore, there's a fair amount of field testing that goes into decisions like this. When the reborn Triumph Bonnevilles were first released, valve inspection intervals were scheduled every 6,000 miles. Reports from the field led Triumph to reevaluate the intervals, so they were extended, first to 8,000 miles, and eventually to 12,000. As to why there is such a huge disparity between the 1300 Star and the 950, I can't say, but Yamaha must feel it's necessary, as the need for the more frequent adjustment puts the bike at some maintenance disadvantage compared to its competitors.