Dave pretty much covered it.
Lets be sure you got it.
Go back to my mark up of MAD's diagram 'B'.
Trace your finger over the purple arrows from the alternator to the main junction.
How many connections?
Did the wire size drop from 12 gage or 10 gage to a smaller wires along the way?
Again. How did your finger cross the ammeter while following the wire from the alternator to the main splice?
Lets do one scenario at a time.