If you have not charged the battery in a while and there is a drain on the battery, (such as your door open and the courtesy lamps are on). The voltage may drop below 11 volts or so, but could still be plenty to crank the starter.
For troubleshooting purposes, 10-12 volts would be good. That is why I mentioned a test light. If there is enough volts to light the lamp in the test light, find the next point down the line and check.