Gary, I don't know why, but I have seen it and it seems to relate to certain bullets and powders in certain barrels.
My 260 has a LW match barrel, 1450 weight, Saami min spec chamber and a really short throat. I spec'd it to shoot 120gr BT's. Bullet is in the lands at about 2.5mm short of magazine OAL. When cleaning the barrel bore, it seems very smooth.
It won't shoot for s**t with H4831. With Varget I get very good accuracy, but velocity is not so good, 100 - 150FPS short of the book.
However, change the powder to Benchmark, or Reloder 17, it is a different beast.
With Benchmark and the 120gr BT's, it is hitting book max velocity (2900FPS) at about 1.5gr short of book max. It is badly over pressure at book max powder loads, flattened primers, extractor marks etc. Reloader 17 gives almost identical results with an extra 1.5gr's of powder.
Load a 100gr BT over Benchmark or Reloder 17, it turns into a pure animal. In excess 3300FPS, again with 1.5gr short of book max loads, again this is max load in this rifle. As you approach max powder charge, pressure goes mad. That is kicking the tripe out of a 25/06 with an '08 case. I am actually backing this load off, as I think it is too fast for deer. Confirming trajectory, I found a 2cm high zero at 100m gave a 2cm low POI at 200m. That is seriously flat shooting.
I should say all this is with a 20" barrel. I had 2" knocked of the barrel a few months ago for the net loss of about 40fps per load.
My conclusion is that this is as a result of a good quality barrel, combined with a tight chamber, short throat and fast for cartridge powders.