Someplace in my office I have the exact mixtures/chemical composition of most of the commercial black powders. They have higher/lower moisture contents, different amounts of the three basic ingredients, slight variations in granulation size and other variables.
Given the above, the burning characteristics will be different from brand to brand. It's probably not fair to say that powder x is better/worse than powder y, but rather just different.
Swiss may simply burn drier and thus produce harder/drier fouling (potassium carbonate).