If there are no enemy ships, _nextShipCooldown increments. If you call an enemy ship during this time period, then _nextShipCooldown is nonzero for the next time there are no enemy ships. This means that the next ship will spawn sooner than intended.
If there are no enemy ships, _nextShipCooldown increments. If you call an enemy ship during this time period, then _nextShipCooldown is nonzero for the next time there are no enemy ships. This means that the next ship will spawn sooner than intended.