Ben at the risk of oversimplification, may I suggest looking at the subject in a layman’s manner? Think of electricity as “stuff”. Actually electrons, those little pieces of stuff that whiz around an atom as planets whiz around the sun. Think of a condenser as a closed can with a wire leading into the can. Magnets passing by a coil of wire drag those electrons out of their atoms, shove them through a circuit and back to the other end of the coil when the point are closed. When the points suddenly open, the electrons want to keep moving for a short bit of time. They will try to jump the gap across the points, making an arc which burns the points. A condenser gives them a place to go instead, that place being the “can”. They are crammed into the “can” by whatever pressure is driving them. That pressure is what we call “Voltage”.
Now, comes the matter of “capacity”. The capacity of a condenser determines how many electrons can be crammed into it at a given voltage. It fills up with electrons till the voltage inside the can equals the pressure pushing them in. This takes time. Too big a can takes too long to fill, and the voltage doesn’t rise enough. Too small a can and it won’t hold all the available electrons. Add to all this, a leaking condenser spills some of the electrons out to ground. Why does all this matter anyway? Because at this critical point in time when the points open, the polarity of the coil changes and the stored electrons flow back out of the can, in series with the changed voltage in the coil. This adds to the spark obtained in the secondary winding of the coil.
Bottom line is, there is no magic number for the capacity of the condenser. A lot depends on the strength of the magnets dragging the electrons, the size and magnetic properties of the coil, the RPM or speed at which the magnets pass by the coil, etc, etc. In other words, the electrical engineers that designed the system arrived at a best value for the system under all or average conditions.
And that’s what it is, the “average”. Notice there isn’t much difference between the smallest and larges capacities listed.
