In all likelihood, it has more to do with the specific batch than it does the skirts in particular. It's tempting to look at the "good" pellets and compare them to the "bad" pellets and ascribe the fault to the most obvious thing that pops out. It's the old correlation doesn't equal causation dilemma.
I used to look for the same correlation with head sizes. It just so happens the 18.13gr JSBs are the pellets I've used more than any other, and my very first PCP produced amazing groups with some that measured 5.52mm. When I ordered some more, they also measured 5.52mm but they didn't group well. The very same rifle today is grouping just as well with some that measure 5.47mm (in case you're wondering, the barrel isn't choked).
The same can be said of thick skirts and thin skirts. Some batches with thin skirts have grouped well, others terribly. Some batches with thick skirts have grouped well, others terribly.
I've had all kinds. The ones that are terrible in rifle A may produce amazing results in rifle B or rifle C.
As good as JSB is, they don't seem too concerned with making a consistent carbon copy of a reference design. I frankly don't understand their strategy unless it's to produce just enough slight variations that it gives us an opportunity to find a batch that works well for the many different barrels that are out there. But whatever it is, it works.