The listed Voltages are the Minimum Voltage (or Potential) between the two spheres, where the Dielectric Strength of the Air will breakdown and allow an Arc to form between the two "Electrodes".
The Arc leads way for a Plasma to be created, if there is sufficient Amperage available to create one.
The size of the Plasma, its intensity, and such, depends on the available Amperes which may be transferred across the gap - and in the long run, the total amount of Electrical Power available (in both True and Apparent forms - more accurately, total Reactive Power for an A.C. system, or simply True Power for D.C. systems).
Once the Plasma is formed and "Somewhat Steady" Current is flowing, the Potential Difference (or voltage) between the two spheres becomes reduced, as the Plasma is highly conductive (ref. "Negative Resistance").
This is getting kind of complex, so I will stop here
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