There are small differences in down feathers. For example, Eider is considered best and rates a full 900, BUT it only lofts to 800. It is just a better insulator than goose down. Down feathers among species will vary. Average age of the bird. Average weight, amount of fat in the diet, average years temperature, etc. These all effect the down for that year. Usually not that much, but, if you are looking at 700-750 fill down (EN measurements,) you can notice it. This is about 80 points or so. Soo, if one large batch is labled at 750, it can actually be between 670 and 800 (the max by european measurements.) Fill and loft make up the largest part of a bags warmth, but there are other things going on.
For example(not the only one): Oils in the feathers can vary, causing an ounce of 750 fill to be warmer than an ounce of 800 outside of a lab. The 800 cluster looses too much because it is too fragile to suport itself with humid air. An oilier cluster in the 750 does not degrade in the same humidity. (Just one example, the next batch may be different.) Soo, depending on where you hike, you might be better off with a so-called lower fill number. The smart camper knows the difference.
There are simply too many variables to say anything about two individual bags that differ by .3" of loft. Well within expected variability simply due to storage differences. Maybe one bag was under the other, as an example.
BTW, I have an older UL Super Stretch Down Hugger 0. I find that the length is very generous for me at 5'9". Warmth is good to 10F (the coldest I have had it out to.)It is rated to 6'.
But, I believe the author should check with MontBell before saying an "850+" fill upgrade would be good. It may not be possible if they are using the Euro system, as I think they are. It will just encourage them to start playing games with advertising numbers again. Other than that, a really good review.