Two deep-cycle batteries connected in parallel by a single terminal
A 12 volt battery has 12 volts between its terminals.  Voltage is always measured between two points - there is no "absolute" voltage.

If you have a 12 volt battery and an 11 volt battery, and connect the negative terminals, the voltage between the positive terminal of the 11 volt battery and the positive terminal of the 12 volt battery will be +1 volt, assuming the negative lead of your meter is on the positive terminal of the 11 volt battery.

If you connect the positive terminals instead, there will be a 1 volt difference between the negative terminals.

The term "Ground" is much misused in the electrical field.  Occasionally it does mean a connection to the Earth, but most often it is just the point in a circuit that the designer chose to call "zero volts", and uses as a reference when measuring voltage elsewhere in the circuit.