Because your observations are quite valid..., for what you've shot and what you've seen and experienced.
All we are doing here is proving that flintlock locks and rifle barrels like different things, even when they appear to be identical, eh folks?
4F is often very slow for me..., What?? You see folks we get a lot of ground fog, and misty mornings, here in Maryland where I hunt. The deer and the squirrels and bunnies are found in areas near some water, and I've been in the woods in fog when it's noplace else but the creek bottom.
I've found that with my locks, be they American, Italian, or India origin, the 4F seems to like to soak up moisture, while using 3Fg for both prime and main charge, means no worries. Range time is hunting accuracy practice time for me, so I use on the range what I use in the woods.
3Fg for me.
LD