When you create an event, by default, the owning calendar is the active calendar at the time you clicked Add Event. Ownership of an event is important because:
To change an event's owning calendar, select the new calendar from this drop-down list.
(Note: Alternatively, you could publish this event to appear with its current owning calendar plus multiple other calendars using the "also shows on" or "mix-in" features.)
Learn more: Also Shows On vs. Mixing In