It is a similar test, but not the same. From the battery positive to the starter tests the usually-red power cable. From the battery negative to the starter case tests the ground path; which includes ground cables, conductivity between the engine and starter, etc. Both readings should be "near-zero" while cranking (well, always actually). Your battery is at 9.8V while cranking, but voltage from the starter "power in" terminal to the starter case is only 6V, correct. Let's find out if that 3.8V loss is in the + or in the ground, then you know which one to focus on. Worst case... both have a couple of volts loss.