Short answer: the higher voltage overcomes the denser mixture encountered at the top of the compression stroke.
The strike voltage will vary based on the engine's operating state. At higher load (high BMEP) the voltage increases considerably: an engine that misfires will usually do so under wide-open throttle. High-performance engines will thus use even higher voltages - 50kV or more - to avoid misfires (MSD, Accel and others offer a whole aftermarket based on this.)
I once worked on a system for V-twins that varied the voltage based on engine load. The idea was to use a lighter spark at light load and thus use less energy. In retrospect it didn't make much sense since having a hot spark at light load does no harm (for this, and other reasons I didn't waste a lot of time on it and walked away from the project.) Nonetheless you can buy aftermarket systems that do this from an outfit called Jacobs, who make lots of claims about increased performance / mileage / time between tune-ups, etc. I think it's a lot of baloney.