Alternator only charges when AC is on.
 

You have variable charging system.
The voltage will fluctuate depend on the status.
With fully charged battery and no high demand, you will have lower voltage.

Frankly I would like to know short answer for it as well.
Driving diesel, I have ScanGauge for monitoring DPF and having 4 displays, monitor volts as well. Having 2 of W211 they show differently, but the one I like is charging at 14.7V during braking and then goes below 13V on acceleration. The alternator power draw from the engine is opposite to voltage raising, so that is whole idea of the program, that regenerate energy and gives cleaner idle when alternator is not adding load.
Other car voltage change makes no sense at all, but the battery in it is older, so that might screw the program.
I recall that W211 can display voltage on cluster, so you can do that without special gauge.
My understanding is that when you see 14+ v during drive and battery is low after, than it is most likely battery dying. How old is it anyway.? In those cars most batteries go in 4 years.

After you jump or charge battery can you drive car?
Will it restart with in few hours?
Does it die after so many miles while driving and battery dead?
If you jump car and then can drive forever your alternator is fine.

If battery was dead or had dead cell - alternator would try max output to try and charge dead battery.

First test battery. using battery load tester with battery out of vehicle
IF battery OK Then check for parasitic draw with key out.

But to test alternator you need to remove from car and put in tester at autoparts store.

Autopart store testing variable charging?
You are not serious?
Happy New Year

Maybe this??

They do since pretty much all cars have that today.
It tests alternator only not the complete system as that is controlled by car computers

Gm has been doing since 2007.

Update