Why is it important that the interior surface above the calibration mark of a volumetric flask be absolutely clean?

Why is it important that the interior surface above the calibration mark of a volumetric flask be absolutely clean? What error would be introduced in the concentration of a solution if water beaded up above the calibration mark? Would the solution be more concentrated or less concentrated than expected? _______________________________________________________________________________

Zumdahl, Steven S.; Zumdahl, Susan A.. Lab Manual for Zumdahl/Zumdahl's Chemistry, 9th (Page 375). Cengage Textbook. Kindle Edition.