I believe you will find that the extra 1" lofting difference of the 750fill down is actually about the same warmth as the 850fill down.
Usually higher fill downs have larger and better developed down feathers than lower fill downs. This means that the air pockets are actually smaller, thus more efficient at stopping convective currents through the down. I am not sure of the exact numbers, but using proportions based on 900fill as the maximum you have 750/900 compared with 850/900, or, .8333 and .9444 for relative warmth, hence, relative heat retension properties (assuming 900fill down would be 1.000.)
Soo, to retain the identical warmth of 900fill down over 4" of loft, you would need about 4.2303 inches of 850 fill down or about 4.8000 inches of 750 fill down. (This is the inverse of the heat retention for four inches of down insulation.)
Given the differences in down rating is not absolute (it varies by year and batch) and the weight of the shell layer compressing it slightly, it is reasonable to assume that the 1" you are seeing is about correct if they are both the same temp rating. For example: The 750 could be an actual 725 legitimatly for a 4.9655 lofting, for example. The 850 could be an actual 875 legitimatly for a 4.1142 lofting, for example.
This assumes you maintain a rather exact equivalency in fill amounts in each, of course.
I would rather think the one inch difference was through intentional calculation (like the above) rather than any real error. Note that 1 inch in 2 inches of loft is a LOT. But, I am guessing they would be of similar warmth, give or take.
