Whilst I would always take the specs from such literature with a pinch of salt until I've verified it myself, the reason will likely be because the "Easy Servo" steppers have feedback (from the integrated encoder) around the motor. Normally with a stepper motor you have to allow a fair margin of torque to make sure it doesn't miss steps when presented with a "heavy" load. By monitoring the exact rotation with the encoder, the driver can detect missed steps and keep on pulsing the coils until the motor gets to where it ought to be.