Airplanes are not airtight, so the system is designed to continue to pressurise the cabin as the air escapes. As they didn’t go above 14,000 feet, I’m guessing

Airplanes are not airtight, so the system is designed to continue to pressurise the cabin as the air escapes. As they didn’t go above 14,000 feet, I’m guessing it could keep up with the pressure loss from the windows.