Internal Motor Encoders Not Reading Defined Gearset

This is our teams code (Note this is in a header file):

static Motor left_mtr_back(1, E_MOTOR_GEARSET_18, false, E_MOTOR_ENCODER_ROTATIONS);
static Motor left_mtr_front(13, E_MOTOR_GEARSET_18, false, E_MOTOR_ENCODER_ROTATIONS);
static Motor right_mtr_back(9, E_MOTOR_GEARSET_18, true, E_MOTOR_ENCODER_ROTATIONS); 
static Motor right_mtr_front(12, E_MOTOR_GEARSET_18, true, E_MOTOR_ENCODER_ROTATIONS);

We include the header file across all our code so we can use the motors anywhere we want. The only problem is that we have defined the Motors with E_MOTOR_GEARSET_18 (green gearbox) and left_mtr_back is reading as if we have defined it with E_MOTOR_GEARSET_36 (red gearbox). When we lift the wheels off of the ground and run it, it reads as 100RPM instead of 200RPM. Does anyone know why this could be the case?