Perhaps the lupulin gland oils that are dissolved at 150F vs boiled at 212F react to form different compounds that affect the perceived bitterness in various ways. There have been studies on the effects of temperature on alpha acids in the lupulin glands of hops. I would be interested to understand the findings, but it's $30.00 per study.
This is what I've always heard and gone on. It seems to be backed up by this...