That measurment was taken from the + on the starter to ground and the + on the battery to ground, same voltage both times, while cranking. that's about the usual amount of drop i get on other small equipment.
you know, thinking back a little bit, i wonder if i ought to try shimming the starter back a little bit, because the way the starter system works, the gear is connected to the solenoid, so when the solenoid pulls back, it pushes the gear outwards, and i wonder if it's too close to the flywheel gear it won't spin to mesh because the gear is hitting so the solenoid can't make the contact to pass power through the starter.
BUT, if that were the case, why does it work perfect 60% of the time?