the answer is pretty simple... but the majority of folks are going to be in the "just don't get it" category... but here goes anyway... :o
It's a barrel time
issue. The actual time the bullet spends in the barrel has EVERYTHING to do with how accurate you shoot.
A fouled barrel will result in more friction and slower barrel times.
If your use my method of load development (OCW load development, see link at bottom of my post), you'll come up with a much more stable load that will shoot well longer before a deep cleaning is necessary.
Also, as mentioned already, the fast 6mm bullet from the 6/284 is not a .308.
It's going to shed copper faster, and all else being equal foul the barrel faster.
If your load is well developed, you will find that your cold, clean bore shots will touch, or nearly touch your follow up shots, and you'll shoot longer strings with good accuracy before you need to clean.
Realize too, that your barrel is getting hotter with the 6/284 than it will with a .308...
The diminutive, pedestrian, ho-hum .308 still has some advantages to offset those often touted disadvantages... ;)