I think are getting confused because there is no difference in measurements between 0 resistors and 1 resistor. That doesn't mean if you add more resistors, the voltage will stay the same across one resistor. If you have two, the voltage stays the same across both taken together but is divided between them individually.
When you add a second resistor, the difference of potential on the first resistor will have to decrease because you now have a voltage across the second resistor that you have added to the circuit. If the voltage (12V) didn't decrease across the first resistor, then that would mean you would be measuring 12 volts across both resistors. If the voltage didn't decrease across one resistor when you add more resistors, then you might as well say that the voltage would not decrease across any resistor. This just doesn't happen. You would effectively have two potential differences of 12V in series which would total 24V. That would be quite amazing as you have created an extra 12 volts out of nothing.
Look at it this way: When you add the second resistor, you will have less current. There will be less electrons flowing in the circuit. That means that the potential differences across each resistor will be less. E=IR. If you have less current through the same resistance, the voltage will be less.
I guess the real question is 'why do resistors develop differences of potentials in the first place?' Well consider a simple circuit with a battery, a wire, and a resistor. In the wire there are more free electrons than in the resistor. When you close the circuit, electrons will flow. Because less electrons can flow through the resistor, you end up with an excess of electrons on one side of the resistor than you do on the other side.