My dad has a lung nodule that has been stable for 5 years. this year the ct scan showed it grew 1mm. can ct measurements be wrong?

Yes, it is not exact: Growing lung nodules can be cancer. But the growth has to be real which means increase of more than 2-3 mm before we can get concerned. The cross sections on ct are rarely at the same level so there is one or 2 mm variation between serially done ct scans.

Yes: Ct measurements are 2 dimensional measurements of 3 dimensional objects so depending on how you choose to measure an irregularly shaped nodule will alter the resulting measurement. A 1mm change is probably within the allowed "error of measurement" for the test meaning that the test may be wrong about such a small change. At 5 mm the nodule can still be watched for growth with a later ct scan.