Today, it's proabably the carbon...that's the only part that can't be chemically pure today as "charcoal" isn't chemically pure carbon (and BP works best when it ISN'T pure). Even today, some will specifiy a certain wood charcoal, others will just say "hardwood", and many just buy the charcoal from whomever is selling it as a "generic". Adler and Willow seem to have been the most prefered wood...but I'd guess the local makers of the 1800's would use whatever grew near them.
The glaze may have something to do with it as well...that thin layer of graphite (and perhaps other chemicals) that gives BP that look. Have lave seem some older powders that didn't have that glossy black coat, and others that were so rounded and glazed over that it looked more like smokeless. Would think that would have more to do with igniton, but it would have to effect the bruning as well.
Will read about "soft" and "hard" black powders of the past...some would crush easily and others would resist compression.
Were some other variations in the past...and a belif that powder stored well over the years actually got better (would identify the best lots and store it away under the best possible conditions for use in powder trains and the like).
Reading on the problems in the 1970's when several makers stepped in after Dupont stopped making BP (at least for a time), there were some granulation issues. Even today, one makers FFFG isn't quite the same as another's.
So, even today, with all the chiomically pure ingredianets, we can't get two makers of BP to end up with the same results...that would have been even more so in the 1800's, and each powder would have had more variables built in. Evidently, from the books of the time, some of those combinations turned out pretty well.
But would there be a commercial market big enough for "botique" black powders? One specifically fomulated to work best in shotguns...or one specifically best at small bore rifles?