I agree with LD on this one...it's no real big deal, IMO.
In fact, sometime before Goex came out with "Cartridge" grade BP, it was quite common for BP cartridge shooters to mix 2Fg and 3Fg on a 50/50 basis....other, more exotic showed here and there..like 60/40, 75/25, 80/20, and God only knows what else, but it turns out it was nothing more than shooters having fun and doing what shooters have always done.
This "mixing" is said to have come about due to the lack of the old balloon head cases which actually held a few grains more water / powder...today we have much better / stronger cases but it's darn near impossible to seat a wad, a grease cookie, and a bullet over 70gr BP in a 45-70 case....without the use of a compression die.
Anyway, Goex quickly latched onto this practice of "mixing" 2F & 3F by the old timers, and came out with what one might describe as 2.5Fg calling it "Cartridge grade" and charged $3 more per pound....there may be a wee bit more to it than that, but that is the basic story.
I shoot, and have been shooting, Cartridge Grade in all my muzzle loaders for some time now, mostly because I won a case of the stuff years ago and still have several pounds left...if it is any better in a muzzleloader than 2Fg or 3Fg ie, more accurate, less fowling, whatever, I am not astute enough to see that difference....but still yet it shoots just fine.
Uncle Russ...