I'm replying once more....because I did a little looking around and comparing fill powers in sleeping bags and temperature ratings. Within the practical ranges of fill power there seems to be a direct correlation between warmth and fill power, all other things equal. More density does not necessarily equal more insulation power (think of heavy synthetic insulation as an example). I looked at seven sleeping bags of comparable design and the same rating and I found that in fact in every case higher fill power resulted in a lower temperature rating. (spreadsheet available on request).
Like you, I had thought perhaps my old low fill power jacket was warmer. It might have been, but it could have been just as warm and weighed a lot less if it had higher fill power down. I don't think there is any getting around that.