PWM duty cycle meaning.. how does it affect current through motor
When PWM is correctly implemented (i.e. high enough frequency...) it is electrically equivalent, from the perspective of the motor, to changing the voltage across the motor terminals. The justification for why this is true is explained very well in this answer: Is there an ideal PWM frequency for DC brush motors?. As mentioned in some of the comments, you are likely observing this effect because at a lower speed your motor is encountering a greater load than when it is spinning quickly.