I have to think about this, So, the water analogy, If you regard the 13V Dynamo output as the Head of Water. The current is the flow rate. If the Basin (Battery) is full, the water will not flow (0amp). If however you drain some water, more water will flow.+ XAmp. You can still measure the head ( Volts, AKA Potential Difference) of water even though no current is flowing. In order to overcome resistance in the Battery (the pipe work) the difference between 12.5 and 13 V creates the means.
Simplistic, But is the way I view it.
Pete