3.5grams? Whoa, isn't that the max astm? If memory serves me correctly, that would be heavier than any other RPS paint that's been measured.
Was all the gelatin based paint in sealed bags prior to measuring (trying to rule out the chance of the balls soaking up water, influencing their weight)?
I'm also assuming you had a typo when you said the RPS Premium only weighed .3grams more than the Hydrotec when your numbers seem to indicate .7 to a full 1 gram weight difference.