you do NOT recalibrate between tanks, certainly not based on what they were saying. You calibrate once, and you wait until the reading stabilizes, that may very well take 15+ seconds depending on the o2 sensor, but you have to be patient.
True. The whole issue would be avoided if people would slow down and give sufficient time for the gas in the tank to fully replace any residual gas in the analyzer. So you're right in that the real issue isn't "recalibrate or don't"; the real issue is to take the time to analyze properly.
Unfortunately some people (like the guy that analyzed my 8 tanks, and probably more that day) get complacent (because they do it every day and never have a problem) and rush.