I've never given a lot of thought to this, but this seems most likely to me. I shoot a lot of 308 and have found it to be very easy to find a good load for. Basically, every powder I've tried gave me groups that were acceptable for my needs. My preferred powder choices are based on getting the speed I want and temperature resistance.
I had a 7mm Rem mag for a while and was never able to get a load I liked even after trying several powders. Most of them were not only mediocre accurate but were close to 150 fps slower than what I was looking for. I finally tried Reloder 25 and got the speed I was looking for, but even worse accuracy. I'm sure that if I'd kept experimenting, I would have found what I was looking for, but I have better ways to spend my time and money so that rifle was sold.
And I also had a 300 WSM. I found a very accurate load of H4350 that was pushing a 180 gr bullet to 2950 fps. That made it easier to sell the 7 mag. After reading that others were getting closer to 3050 fps using Reloder 17 I decided to give it a try. I don't recall the exact charge weights, but I was about 2 gr below max and had already reached over 3000 fps. Accuracy was poor. Only 1/2 gr less resulted in the same 2950 fps and accuracy I was getting with H4350. Had I gone to a max book load I'm sure it would have been an overload in my rifle. I decided to just stay with H4350.
It is very possible that the belted magnums had very long throats, given the sort of people who buy the things, are looking for maximum velocity, not maximum accuracy.
I have one 270 Win that has a match grade barrel and a match chamber. It shoots very well. My FN Deluxe in 270 Win, with its commercial chamber and throat, shoots looser groups than the rifle with the match chamber. I believe the tighter throat reduces bullet wobble, but, I can push bullets faster in rifles with longer throats before the primers pop. Thems the trade off's. I am certain the more recoil a cartridge produces, the larger the groups will be in a hand held rifle.
It was well known, back in the day, that very accurate ammunition was easy to create inthe 308 Win. With good bullets from 125 grain to 175 grain, if you used IMR 3031, IMR 4895, or IMR 4064, and your rifle did not shoot MOA or less, it was not the fault of the bullet, powder, or rifle.
A 308 Win is a positive pop gun compared to the recoil from the big belted magnum. I can say, this month I took my 300 Win Magnum out, and around shot three, I was squeaking after each shot with 150 grain bullets. And squeaking loudly after shooting 190's. Regardless of the mechanical perfection of your thunderstick, recoil is not conducive to accuracy when a human is pulling the trigger! I quit shooting after 30-35 rounds as all the fun had gone.
Just last week, I got to shoot a Colt 22lr ACE pistol.
This conversion unit was designed to simulate 45 ACP recoil with a 22lr. I told the owner I thought the whole idea was stupid. Firstly, I don't like recoil, and secondly, if you want the pain of a 45 ACP, just fire a standard 22lr, and have a buddy hit you with a canoe paddle after each shot!