Need Help with 'Couldn't have understood any better'

What does that exactly mean when someone says "I probably couldn't have understood any better"?

i. The voice(tone) was really great (as opposed to how the robotic voices are usually) that I could comprehend everything

ii. I couldn't understand anything because the voice was so bad

We were actually watching Google Dupex demo on how the AI makes phones calls for the owners and schedule something.