To check the accuracy of a graduated cylinder, a student repeatedly filled the cylinder to the 25-milliliter mark using water delivered from a buret and then read the volume delivered. The results of the trials in milliliters were as follows: 1) 26.54, 2) 26.51, 3) 26.60, 4) 26.49, and 5) 26.57. What type of error do these results primarily demonstrate?