When the system is busy recharging the battery (like after a start-up) it'll read a lower voltage than when it's all topped up. And as everyone says, the volt meter tends to show a different voltage than actual - usually a volt or so less - being several connections removed from the battery, plus inaccuracies in the meter itself.
It's not the high voltage reading that matters so much as the current. But we don't use current meters so can't monitor that. I've had a system that charged at 13.5 without problems because it had very good connections with plenty of current flowing. Remember that Voltage is the potential, but Current is how much can flow. For example, a cup of water dropped from 176 feet has the same potential as Niagara Falls, but nowhere near the same current.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*