I am using grove ultrasonic sensor for measuring object in mm but i am getting not accurately some time it shows like i keept object in 6cm it have to show 60mm but it showing 54mm please suggest me i am not having data sheet also because I am stucked in

I am using library code and modified like same cm code I am using mm formula

Can you show the setup for your experiment?Transmitting and receiving of ultrasonic location slightly than module base to advance some so some error.
In addition, also affect the ultrasonic distance algorithm