Now the truth... All the rubber comes from the same place. GRP does not produce the rubber. It all comes from Japan. There are many factors that could affect performance such as wheels (already mentioned), age of the tire (a 35 from 6 months ago may durometer 40, but it still has the characteristics of a 35), inside diameter of the donut when mounted, heat introduced into the tire when trueing, etc. My guess is that it is either the wheel or you may have one brand that is "fresher" than the rest.
Everyone has heard the "we have our rubber made just for us" story from the various manufacturers. When you hear this, know that you have been lied to.