Ulstrasonicsensor SKU_SEN0307 reads wrong values

I have got the following sensor: https://wiki.dfrobot.com/URM09_Ultrasonic_Sensor_(Gravity_Analog)_SKU_SEN0307

So its an older version of URM09.

I copied the code from the link and tried it on my arduino uno. It measure the following distances:

142,cm 143cm, 144cm

These distances are wrong, because I put the sensor at a distance of about 4 cm…

Does any of you have any idea what could be the error here?