There is so much variation even station-to-station. One petroleum engineer put it to me, go to the station that is the busiest/most popular...the in-ground tanks are very prone to water contamination, and the stations that don't refill as often are the worst offenders. This worries me with 93-octane fuels in the Houston area, as it may sit for a very long time. I try to use Shell V-Power 93 only, and have found a few specific stations that are the least problematic. When racing the car, however, buying Shell URT100 or URT105 in a 5-gallon pail has been the safest bet, and the car runs great with that fuel in the tank.
YMMV. Great data points from you though. I would like to see the same thing from different stations (within the same brand), and possibly from different distribution companies (outlying towns, etc.) to see if there is a difference.