Battery Voltage Question (x2)

I am posting this seperately from my larger post in this forum concerning my no crank issue (Run Mode Change System Check). This is purely

Battery related:

Brand new Red Optima battery showed 12.7vbc yesterday when I installed it. Today it shows 12.2vdc after running my backseat reading light for about half an hour and trying to start my no crank car 20 times.

1. Does this suggest a parasitic drain?

As of ten minutes ago, I see that 12.2 vdc across the positive and negative battery terminals, with cables attached. But if I disconnect the negative cable and measure voltage between it and the negative battery terminal, I see

11.7 vdc.

2. Shouldn’t I see the same 12.2 vdc on the negative side?

Thanks for your insights.