You're to be congratulated for the thorough in your investigation. Good work.
Not knowing for sure, I'd say that your results may well show that the trend is more closely related to barrel temperature increase than anything else.
I'd say that you have found the reason that heavy barrels are used for more serious long range shooting.
However, for LRH its the cold bore shot that counts. Your rifle seems pretty fair with that.
I recommend giving up on more testing and go harvest something.