Various programs or spreadsheets for reducing photometry are sometimes described as "based on" Henden and Kaitchuck or Hall and Genet. But these references are not software functional specifications. They leave implementation details open to interpretation (or imagination), and different implementors can come up with software that gives different answers. I would like to offer PEP data reduction as an example. For those not familiar with PEP, it is single-channel photometry. Only one star can be measured at a time, and the AAVSO standard sequence is: comparison star, variable star, comp, variable, comp, variable, comp, check star, and comp. The reduced variable magnitude is an average of the three variable samples.
The AAVSO web program for reducing PEP data is WEBPEP. It operates as follows:
1. Reduce all net star counts to instrumental magnitudes
2. For each variable sample, time-interpolate a comparison instrumental magnitude from the bracketing comparison magnitudes and compute the differential instrumental magnitude.
3. Correct the differential instrumental magnitude for extinction.
4. Adjust the extinction-corrected magnitude to standard magnitude
The single differential instrumental magnitude for the check star is computed the same way, except that no adjustments are made for extinction or standardization.
The three variable standard differential magnitudes are averaged and added to the comparison standard magnitude to give a reduced magnitude. The lone check star differential magnitude is added to the comparison standard magnitude to give the reduced check magnitude.
I have already seen examples of PEP data reduction programs that vary from this algorithm (eg: interpolation done on net counts, not magnitudes; extinction correction applied to the final averaged differential magnitude instead of individual sample magnitudes; extinction applied before interpolation). I'm not saying these variations are wrong, but they are different and will give different results. It would not surprise me if the various CCD observers are likewise using reduction software with different implementation details. As we try to squeeze the highest accuracy out of AAVSO photometry, we should keep in mind the existence of systematic effects introduced by software.