its just a differant means of measuring the force, what your missing on your '50nm and 80nm' is that its rated per second iirc, i.e

50nm/s not just 50 newtons.

and the break torque is just what the diff will start to turn at against a fixed load. i.e 9 kgfm will be the point at which the diff will turn against itself.

hth

mark