In addition to the analyte you are trying to measure, many samples contain additional elements, such as proteins and lipids, which can interfere with the free association of the target analyte and the specific antibody in the kit. In the standard curve wells, the pure analyte is simply dissolved in assay buffer, so those potentially interfering components are not present. When the assay is performed, you may encounter a situation where the sample well and a standard well contain exactly the same amount of analyte, but additional components in the sample well interfere with binding to the specific antibody. As a result, these wells will generate different amounts of signal even though the amount of analyte is the same.
