Charging system info non mopar

Check your wires going to the starter. Just an idea I'm throwing out there, but I've worked on more than one GM in which the positive lug is a tad loose, causing a voltage drop. (A lot of GMs route the alternator through the starter.) Since your tester is hooked to the battery and not the alternator this could explain the variance between your tester hooked to the battery and your direct alternator test.