What is the Ah to voltage relationship of a 12 V lead acid battery?
There is no co-relation between a battery's remaining capacity and it's open-circuit voltage.

The discharged battery's no load voltage could be above 12 V but its internal resistance would also be high.

Without the required starting current, the motor would remain stationary and present itself as a dead short across the battery terminals. The battery voltage would be close to zero under this condition.