I also know from personal experience that an oddball loose fitting pellet will be a flier. Back when the boxed Crosman Premiers had QC issues (around die #7) I would toss out a CPL that was undersized and visually dropped too deep into the .177 R9 leade and reload another. If the replacement pellet visually sat at the proper height in the leade it would be pushed home and shot.
The reason I stopped using both 8.4 grain JSB Exacts and the 8.64 grain H&N FTT was that the pellet fit from almost snug to LOOSE in my .177 R9 leade was too variable and I got a lot of UNEXPLAINED fliers! Due to the variation of "fit in the leade" for these two pellet brands I literally measured (and sorted) a few thousand. While I'm sure that measuring pellet heads with a digital caliper was/is rather crude, even after developing a "measuring technique", I also found that the sort was accurate enough so the sorted groups did fit the leade consistently............
LOL.....when I tried to head size supposedly 4.52mm JSB Exacts with my 4.50mm home lapped die it was pretty much a waste of time because a large percentage of the pellets already had head sizes less than 4.50mm and very few were even 4.52mm. I measured a tin of 8.4 grain Air Arma dome pellets and found that a much larger percentage than the Exacts actually measured 4.52mm.
Anywhoo.....I do admit that most of my fliers are "shooter induced" considering that I still get fliers using my .177 CPLs that are head sized to 4.50mm and weighed!
While I found the sizing of CPL pellet heads to 4.48mm or 4.50mm was useful to minimize the "sore loading finger" from shoving unsized 4.53mm-4.55mm CPL heads into my tight leade .177 HW95, I also found that weighing the CPLs was a waste of time since the weight variation from absolute lightest to the absolute heaviest CPL was only .38 grain and it's a "pain in the butt" to weigh pellets!
Calibrating my scale with accuracy ADVERTISED to be .001 gram (0.015gr).........
LOL......I also found it interesting that most "7.9 grain CPLs" were heavier than 7.9 grains weighing in at 8.00gr-8.06gr........
Bottom line..........I personally believe that most of MY fliers are due to "trigger slappin' instead of squeezin".