Yes, those are high numbers for a ground circuit.
But, your charging system seems to be doing o.k. by the last readings.
So, two things come to mind:
1. The alternator has "found" another ground path back to the battery. And your designated ground straps/wires are just window dressing, electrically speaking.
2. Your digital meter is picking up Rf from somewhere and giving nonsense readings.
A/C generated noise, maybe? That's a question for brian6pac. Perhaps checking the ground circuit with the test light? as he mentioned earlier. An 8v or 10v drop would probably make the light glow.