Welcome to tfl!
will produce a power factor that is below the recommended minimum for at least one of my pistols.
Recommended, by whom, for what??
The gun maker?? Lyman?? (never heard of them doing so..)
Or the rules committee for playing a certain game? (have heard of that)
Every gun is slightly different. Every ammo combination is slightly different. Everything made by man has tolerances. These factors react with each other and "line up" or "stack" creating the result we get. Most of the time, the result is very similar, creating a bell curve of results. However sometimes the results are drastically different from what is expected, either on the high or low end of the curve.
NO ONE knows, or can know just where YOUR exact combination of gun and components is going to be on that curve, until it is tested.
It is possible your gun and the ammo you made could produce unacceptably high pressures WITH the starting load. Its not common but it IS possible.
Lyman and the others choose a starting load that they expect to be SAFE, in any gun in safe operating condition. But, they cannot KNOW what is safe in your gun.
So, you start at the starting load (or below), load some rounds and fire them, to test, and prove that load is safe in your gun. Once you've done that, you work up in steps, until you get to your desired level, or reach the listed max loads, testing each step along the way, looking for anything indicating you're approaching or at your gun's max safe level.
Your gun and ammo might start showing pressure signs (like sticky extraction, etc) BEFORE reaching the listed max in the data. OR it might not. Their test gun is not your gun.
Extremes are always possible, so you start low until you KNOW your gun is ok at that level. Doesn't matter what somebody else says you need, what matter is what your gun does.
I've seen those differences first hand. Gun A with load XYZ, every thing fine. Gun B, same load, cases had to be driven out of the chamber with a rod.
It's rare, but it can happen, so until you know your gun isn't "gun B" you start at the bottom end of the data.