the potentiometer has a 1ohm boundary. It is not mathematically correct, and not good when using the potentiometer in low-ohmic circuits. To demonstrate the bug, put a dc source across the pot with a value of 1 or 2 ohms. now vary the pot position.... the voltage at the wiper is not correct. Also observe the R1 and R2 values if setting the pot to 10ohm and the wiper position at 0. You now have an 11ohm potentiometer.