The increase is because the grain size is so small with FFFFg. The amount of energy (pressure) produced when Black Powder is burned is related to the size of the grains. Smaller grains have a larger surface area to weight ratio and burn faster. In addition, you are adding BP where none is needed. Pyrodex is not real Black Powder, it is a Smokeless powder that has been altered to behave like Black Powder. Putting Black Powder and Smokeless together in the same round is called making a Duplex load and it is only for very experienced reloaders, not rank beginners.
Why not just put the pellets in the gun and shoot them the way the manufacturer intended, rather than start experimenting before you have even fired one shot?