In my experience of observing and collecting lighting, I have noticed that North American preheat CFL series choke ballasts intended for low wattage PL-S and PL-C CFL lamps tend to have a noticeable hum conpared to other types of preheat fluorescent tube ballasts. Why is this so?

Could be an intention to get as much efficiency with as low $$ as possible (or just the later). Combined with the better processing control of the magnetic materials and their assembly, it made the ballast designs to drive the cores closer to saturation, as that means less turns of wire and/or a smaller core size. That allows either to get lower losses for the same cost, or lower cost (less materials, like the expensive copper) for the same losses, or any compromise between.
And closer to saturation both magnetostriction becomes stronger so cause stronger vibration, as well as fields leaving the ballast and affecting the nearby steel structures, causing extra vibration there as well, which is then the drawback of these more optimized designs.

It is also possible that the fact that 120V series chokes, consumes much more current than the 230V series chokes, because of the low mains voltage. This combined with their design with exposed winding and core, compared to the sealed design of the 220-240V chokes.