That's exactly what the site says about the 9 tooth, more torque. I used the mower yesterday, it started fine. I'm shooting video though every time I crank it.
The size of the gear is determined by the number of teeth
A 9 tooth gear will be smaller than a 10 tooth gear because you are pulling 1 tooth length from the circumference of the base circle.
So to compensate the starter has to go closer to the flywheel
They may have changed the mounting bracket to get it closer
