My digital dash volt readout varies from what I measure with my hand-held digital VOM, by as much as 1 volt.
GM switched (about 1977) from using an ammeter, as a voltmeter is cheaper and perhaps less problem prone than an ammeter. The voltmeter provides an indirect reading of battery charge/discharge, as follows.
The battery is charging if the voltage (electrical potential) at the terminals is greater than voltage produced by the battery. Fully charged car batteries yield 2.2vdc (volts, direct current), per cell, with six cells. Simple addition yields 13.2vdc. A higher voltage from the cables, supplied by the charging system, will cause current (amps) to flow into, and charge the battery's cells. The voltage will read higher momentarily, as Ken said, to recharge after the large power drain of an engine start, bu also to offset other drains like headlights or large fan motors.
Hence, your voltage sounds rather normal, as long as it does not read 15vdc for extended periods. Remember that corrosion acts as an insulator, preventing full current flow from cables to battery terminals, grounds and positive terminals.
One bad cell, of the six, can drain the others and be frustrating to find. Individual cell voltage can be measured with a voltmeter and probes inserted into the cells' 'holes'.
