Arduino Adafruit Motor Shield V2 Stepper Motor Speed (RPM) reversed

Hi Arduino Experts

I've been using matlab to control a stepper motor through arduino, however one strange thing that I don't understand is that the speed of the motor seems to be reverse to the imput RPM value (i.e. RPM = 8 is much faster than RPM = 10, and RPM = 20 is much slower than RPM = 10)

here's my code

a = arduino('/dev/cu.usbmodem14201', 'Uno', 'Libraries', 'Adafruit\MotorShieldV2');
shield = addon( a, 'Adafruit\MotorShieldV2');
sm1 = stepper( shield, 1, 200, 'RPM', 10, 'stepType','Double');