A little apples to oranges...500 amps is a little different than 100 mils..
With a dirty cable, you can get the full 12 volts, but not enough good contact to carry the starting load. Draw has a lot to do with it to.
Right you are Rob.
Also, for the OP's continuing education:
A decent voltmeter has a very high impedance, way higher than the resistance of a corroded battery terminal in most cases. In a series DC circuit the voltage is distributed across all the resistances in the circuit in direct proportion to the percent of the total resistance each individual resistance comprises.
We talk about the resistance of corroded battery terminals in very general terms but how many Ohms is reasonable? Lets say your starter pulls 100 amps. If (and in this case a bad assumption) the combined resistance of the battery terminals was 1.0 Ohm then with 100 amps the voltage lost at the terminals would be 100 volts (see Ohms law.) But wait where did the extra volts come from? You don't get any extra volts and with a 12 volt battery to get 100 amps with a dead short in place of the starter you need a resistance of 0.12 ohms.
What does this tell us? It tells us the resistance of the battery terminals that permit starting the engine or running the headlights is much much less than 0.1 Ohms. A decent meter has a resistance of several hundreds of thousands of Ohms and more likely millions of Ohms. Just for a simple case lets say the meter has a resistance of only a million Ohms so with a perfect battery connection the total resistance in the circuit (ignoring wire) is 1,000,000 ohms. With a super terrible (you can't start the tractor type battery connection) the total resistance in the meter circuit is 1,000,000.1 so just how much effect do you suppose that will have on the voltage reading? Too small to notice!
That is a reason you want to draw a substantial current through the suspect connections in order to measure a voltage drop.
In the case reported by the OP his corroded fuse to fuse block connections were quite high in resistance, high enough to effect the meter's operation and or reading. The resistance of the fuse connection was NOT just an Ohm or three but several (quite a lot actually), a very bad connection.
Those of you following this thread for credit... note: this material may be on the exam. For you auditors, enjoy.
Pat