Here's what I think (for whatever that's worth).
What you're missing in the comparison is this: A bullet loaded from the muzzle is bore size
- groove to groove diameter at best, but most likely slightly larger than land to land diameter, or even land to land diameter or smaller. A cartridge rifle bullet is typically larger than bore size
and is swagged down to fit the rifling upon firing. Generally speaking, the tighter fitting bullet will develop higher pressure and greater velocity.
So, a 400 grain bullet fired from a 45-70 cartridge rifle (with 70 grains of black powder in the cartridge case) will generally achieve higher velocity than a 400 grain bullet fired from a muzzle loader with 70 grains of black powder.