A calibration error could lead to this result, if the pH probe was not properly calibrated before being used, the measured value of pH would be different from the actual pH value. Meaning in this case the student correctly prepared the buffer.
While measuring the mass of the buffer's acid component (for example NaH₂PO₄), the student did not properly set the balance to zero before proceding to weigh the compound, causing the mass readings to be underestimated. As a result, the student weighed more acid than required, eventually leading to a decreased pH value.
