Maybe because the capacitor is not exposed to the full line voltage. Try to find a vector diagramm showing voltages for a single phase induction motor with auxiliary winding.
The starting winding may be a 240 volt winding. If so, it would be fed from one line and the center connection between the 240V/480V run winding.
It is common for 115V/230V motors to have a single 115V starting winding.
Bill
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"Why not the best?"
Jimmy Carter