As Caddyman pointed out, the alternator is a three phase generator with a diode rectifier bridge. One of the reasons that three phases are generated is that the cosine of 60° is .5. That means that as one phase is 60° past its peak, the next phase is reaching 60° before its peak. Both are at their half power points. .5 and .5 added together = 1, the same as peak for one phase.
This makes for a very smooth output, but not perfect. At each 30° point in a rotation of the generator, the sum of all powers equal zero. At all the other points, there is a small unbalance that results in the ripple. I can’t remember what the unbuffered theoretical ripple is in a three phase generator is but I think the 312 mv in a 12 volt system is not anything to worry about. In fact, 764 mv would not be alarming either.
If you had a diode burn out, the ripple would be a lot greater, on the order of a couple of volts.