AEA What factors can explain these results?

No offence but a lot of times its the loose screw behind the trigger . Lest ways it is in my case .

In my case , when I shank some groups I look at the trigger time & how it's affecting my trigger finger. How is the trigger weight , shape ( sharp edges, etc) & overtravel. This was learned by target handgun shooting , some specialty , and military(bolt) guns. Sometimes a trigger shoe helps greatly on such triggers w/o honing / polishing. Another thing is be consistent in how you hold the gun. I cannot shoot worth a, ya know what, with the thumb up position on any of my guns , feels like I losing grip, which doesn't help the brain concentrating on the trigger & breathing. Other than that,,, practice ,practice ,practice .

In my vast minutes of experience, are your speeds consistent? Is any thing loose, beside the wingnut? Scope on tight? Parallax assuming a FFP. Does this happen with anything but my favorite pellets? I've seen that where the bottom left is the first shot and something in the gun just settles in, then it say's NAH I'M DONE.

Lots of variables at play. What’s your support for the rifle, I’m assuming this is off a bench. Bullpups are tricky and not usually very happy on a bench.
Are you inspecting all your pellets/slugs prior to shooting groups?
Does it do this with a magazine and SST?
Flashing from a destabilized pellet?
Pellet speed?
Does it exhibit the same characteristics at 20 yards?
That’s a few things

The 3/8 and 3/4 groups I don’t see a problem, a 1/8” difference is probably you. The 1 inch group even more so. Shoot three groups of five three times. Make sure wind isn’t blowing. Keep air pressure very close with each group. Same pellets for each. Don’t get in a rush. The problem will show itself if you don’t get in a hurry.

Rick H.

Are you allowing for the fact that for every different pellet tried, it might take anywhere from 40 or 50 pellets to a hundred or two to "season" the barrel to a particular pellet? Airguns have idiosyncrasies that are different from firearms. Then again, some guns are less finicky.