I just came to one systematic influence I have not thought off: in cold conditions it might be that the hand used more frequently might have a better blood circulation. This would result in glucose numbers more representative for the real blood glucose in the main blood vessels. With cold hands we will often see the following effect: we test, then we shake our hands to increase circulation, then we test again and the result is often more than 20 points off. Well, all it needs is some snow in Arizona, LOL.
Good thinking. The other story not told is the fact that our blood system is like a full duplex ethernet pipe system. One pipe inbound and 1 pipe outbound. All sorts of stuff is stuffed on the pipes outbound like glucose, disolved oxygen, hormones, drugs, minerals etc and the inbound pipe returning the exhaust, carbon dioxide, breakdown products, unused glucose etc. Unlike the orderly system of polling, bandwidth sharing, enforced banwidth allocation - token net; things are just stuffed on our veins and arteries willy nilly in no particular amounts.
After heart has pumped the stuff around the system a number of times and liver and gut run out of things to do, random readings at finger tip tend to be stable and predictable. During the loading phase one can get instantaneous readings at any instant that are low, high and heart stopping and then test a few minutes later they are more normal once the rich packets pass by.
In fact, if one watches the CGMS readings versus the caveman fingerstick machine, one notices the cgms is integrating - averaging the readings over time to give one the "AVERAGE" OR MEAN READINGS OVER A PERIOD OF TIME and not the peak readings.
In addition, as this reader comments, activity and use of glucose at any point in time will in fact be going up and down as the skeletal muscle cells store their own reserves locally so that at any given time their may not be an even draw on the blood supply glucose as cells will all be drawing down glucose at different times and randomly so that readings of fingers on hands will vary accordingly.
I doubt that one can really get a consistent reading all over body except when one has been resting, exercising hard and liver/ digestion activity has run out and the saber tooth tiger is not after your butt for a quick snack.