WVMJack, it really is all about the buffering, or lack thereof in pure honey-based musts. Using raw honey instead of filtered helps (as you guessed, the stuff in there pre-filtering, like pollen, misc. proteins, and even some bee bits add a little buffering capacity) but even the variety of honey has an influence on the buffering capacity of the must. Darker honeys tend to carry more of the compounds that ionize in solution, and they provide a buffering capability that the glucose lacks. Still, when I make a pure traditional mead in order to ensure that I'm going to get complete fermentation (especially with a lighter honey such as raspberry blossom or white clover), I'll add some potassium bicarbonate to supply that buffering potential.