Most of these motors(*) have non-integral gear ratios and are not an integer number of steps per revolution, note. They are made for moving car air-conditioning/heater air-flow vanes, I believe, so exact gear ratios
are of no consequence to the manufacturers. The gears are not designed for anything but light duty.
(*) I believe people have identified at least two versions.
There's another thread about them from a while ago: Geared Stepper Motor - Motors, Mechanics, Power and CNC - Arduino Forum