MAD ELECTRICAL?

There's no way to know if the charging is pulsing. You would need an ammeter on the charge wire to know that.
What you're seeing on the voltmeter is system voltage fluctuating.

There are a few possibilities.
The alternator at idle is just at the minimum speed to 'turn on' so voltage is fluctuating between alternator output and battery voltage.

The alternator at idle speed is producing less power than is being demanded.
example:
Lets say the alternator can produce 10 amps at idle;
and the ignition needs 2 amps, the field current 3 amps, and the electric fuel pump 6 amps for a total of 11 amps.
The alternator can't supply the 11 amps so the voltage drops.

example part 2:
Lets say the alternator can't supply the power needs at idle, voltge drops below 13 V or so and the battery supplies a portion of the power. As a result the battery will recharge when the alternator is spinning faster and can do so and if not recharged, it will be trying to suck power whenever the alternator voltage is above the battery's voltage. Until the battery is recharged, its becomes an added demand on the alternator's output.
Taking the example above,
ignition needs 2 amps,
field current 3 amps,
fuel pump 6 amps
recharging 1 amp
for a total of 12 amps.

Ripple voltage which is the left over wave of AC current being rectified to DC. Really shouldn't see that unless a diode is bad.