Intersting stat. Trying to figure out what it exactly means. There are a lot of take-aways.
Let's start with usage which would be Percent sorties per aircraft
Example: A pilot flies 100 sorties and takes a up a P-38J 74 times, the P-38J usage factor is 74%.
But If a few people are very loyal to a plane, but most pilots don't fly it, does the plane have a high loyalty factor? Hmmm. If so usage can be plotted as a histogram of number of pilots versus their percent plane usage, maybe Loyalty could be described by the shape of the histogram. How right shifted the distribution is, would be the 'loyalty' factor.
I think the kills per pilot is a reasonable way to infer this since sorties per plane type is not available in the stats[?] (I didn't check.)
I am very loyal to the P-39, but I don't get a lot of kills, and not many others fly it. So it would have low usage, and low scores per pilot, but maybe high loyalty if one would agree that it has a small loyal following.