If the first shot was from a squeaky clean barrel, it could well be higher than following shots from a fouled barrell, even with swabbing between shots. I find this is more likely, orat least more pronounced,with conicals than with sabots.

I like 5-shot groups. They tend to tell the whole story. Two more shots might have grouped with shots #2 & #3, or they might have been up there with #1, or halfway between. At least you'd know.