KT Eri comp star photometry

I've been measuring some old and new images of KT Eri (Nova Eri 2009) and am using both the AAVSO sequence in the VSP (chart id 10300HXK) and Seqplot for comp star magnitudes. Most of this photometry is from BSM and APASS, with some measures from AAVSONET telescope Wright 30cm. After comparing these sources, I've noticed that there are three distinct zero points for this field. Also, the errors for most of the comp stars are typically 0.05 to 0.10 for stars of magnitude 9 to 14. Many of these stars have been measured on 4 to 12 nights. These errors are much larger than I would expect for such moderately bright stars. When I measure a set of my images for this field, the s.d. of KT Eri is usually 0.01 to 0.02 mag. When I apply the zero point using the VSP or Seqplot comp star mags, the total error is doubled or tripled. I've seen this for other fields as well. Maybe I've been spoiled by the generally excellent field photometry from the USNO 1m over the years, but I'm rather disappointed by some of the newer photometry from BSM and APASS. I'm not sure what to do about this and would appreciate any advice on this matter. Thanks, Bob