pellets are very different to one another.. so many variables it may not be telling you anything. lead mix, hardness, skirt thickness, size, profile, starting pressures all vary and affect power.
However, JSB 7.3, 7.9 and 8.44 are about as similar as you can possibly get, except for the weight variable.
In larger bored .22s, RWS Hobby and Superdome are pretty similar too (we don't care about shape). JSB 13.43s and 15.9s grains are similarish, but the lighter pelles have much thinner skirts.
So in short, unless the pellets are very similar in other aspects, there's too much variablity to draw any useful conclusions.
Also, using pellets way out of the "envolope" for a 12 Fp springer, i.e. 10+ grain .177s or 18+ grain .22 is not useful. Stick with 7-9 and 12-16 respectively.
OTOH, when they are similar, I find the data very insightful...
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.