Why do you theoretically expect one value, with the multimeter you measure another and the Arduino reads a different one?
Welcome to electronics!
As you may know every electronics components has a tolerance, which means, they always have an error associated to it. Therefore, the Arduino might not output exactly 3.3V or 5V. Try and measure it with the multimeter!
As you can also verify, different analog channels (A0, A1, …), give different readings. Why is that? Like previously, the components associated with each ADC (analog-digital converter) - every analog channel has a different ADC - have a tolerance, therefore, they might return different readings. Experiment with different analog channels and verify that they are not all the same.
Now why does your signal fluctuate even though you expect a constant signal? Well, imagine that the Arduino voltage has some small fluctuations and every 10 reads the ADC makes 1 faulty read. If you increase the number of reads from 10 reads per second to 100 reads per second, of course the ammount of faulty reads per second will increase.
And that’s all folks. Here are some tips for your report of L2.