I think that its perceived that some cartridges are LESS accurate than others due to extenuating circumstances. If you were to pick the more accurate cartridge between .308 and 7.62x39, most would pick .308. The problem with the 7.62x39 cartridge is not about inherent accuracy, but the fact that the vast majority of rifles that are chambered for it aren't very accurate to start with. Now throw in the fact that different bore/bullet sizes are used by different manufacturers of the cartridge it could be viewed as a less accurate cartridge than the .308. If some spent the money to build a tight fitting rifle for the cartridge, and carefully assembled ammunition from high quality components, the 'accuracy gap' would probably be eliminated.

ANother example is the 30-30, mass produced lever guns aren't always very accurate, but chamber a single shot pistol or rifle in the cartridge, you can get very good accuracy.

One of the biggest reasons why the .308 is seen as more accurate than the 30-06 is due to the rifle its shot in. In the 1950's and 1960's, the M1 Garand was the predominant rifle used for high power rifle competitions. It was found that rechambering them for .308 made them much more accurate. Its not that the .308 is a much more accurate cartridge than the -06, but a weakness in the rilfe that limited the loadings for 30-06. The 30-06 can only be loaded with a few powders and still be safe to use in the garand in terms of not damaging its gas system. These loads are nowhere near the potential for the 30-06 cartridge and consequently mean less than optimal powder loading densities. The .308 could be loaded to the same velocities as the downloaded 30-06 in the same rifle while having its case nearly full of powder.

If you were to use similar quality ammunition in similar quality hunting rifles the whole 'efficient cartridge' theory pretty much disappears.