VPHOT comparison stars

I am looking at my VPHOT display for a 30 second unfiltered image of CV variable LL AND taken with the Sierra Stars 24" in CA.  Comparison stars go down to and include the 170 comp star.  Yet when I look at the sequence for LL AND, there are three additional comp stars (176, 179 and 182) listed which are not displayed on the VPHOT image.  These three comp stars are shown when plotting a VSP genereated finder chart using a DSS image and a magnitude limit of 19.0.  Is there an arbitrary or an SNR based magnitude limit imposed on the comp stars that are displayed in VPHOT? 

Kevin Paxson - PKV