JM -
THe different number of shots are because I shot them on different days, sometimes at different ranges, and with different criteria in mind. But none of the rifles shot "all over the target". I had a rifle that did that after the barrel went south, but it is long gone.
Eighteen shots into 2" at 100 yards is pretty darn good for a Marlin 375 lever gun, especially when I'm behind the trigger and the sights were factory iron! Nothing wrong with that load or rifle that a better shooter (and better eyes) couldn't fix! Why 18 shots that day? I have no idea! At this point all I know is that it was February 17, 2002 and I shot over the chrono, then came home and recorded the data in my reloading spreadsheets.
The Marlin .45-70 put four into 1.04" at 100 yards, nothing wrong with that, either - especially when 3 of the 4 were in 0.65". But 1.04" hardly quals as "all over the target", does it?
The three shots with the 7mm Mag were because that is how I normally test that rifle for accuracy and zero. More than any other rifle I own, it is a big game rifle, pure and simple - and I rarely take it out except to practice for the annual elk hunt. The reason for 3 shots is that, as a hunting rifle, the first shot has to count. There may be a second follow-up shot. God-forbid that a third shot is needed, but I shoot 3-shot groups just in case. A couple years back (2001) I put 3 shots into 1.9" with this rifle - at 300 yards. (My hunting buddy put me to shame using the same load in his Ruger M77 MKII - 3 shots into 1.0".) Once again, that doesn't even approach "all over the target".
The ,257 Roberts gets used as both a varmint rifle and hopefully this fall as a deer or antelope rifle. Four shots into 1.85" at 200 yards is "all over the target"

Why 4 shots instead of 3? Moon phase, maybe, I don't know.
The .22-250 kind of speaks for itself.
I know a lot of people feel 5 shots is required to determine group size. My own feeling is that the last four are increasingly unimportant. Even prairie dogs don't generally let you get more than two or three shots before they hole up, and big game doesn't generally wait that long before seeking the next county. Three shots is plenty for my purposes.
So, to answer your question as to why the different number of shots? Because! :lol: