I don't think Sierra has ever used SAAMI spec pressure/velocity barrels to develop reloading data. All I've seen in their published data are firearms. Sometimes commercial ones and occasionally custom ones. Their current .308 load data lists a Savage 12VSS rifle with a 26" barrel and 1:10 twist. I've an older manual that lists a Winchester 70 match rifle with a 26" barrel with a 1:12 twist. Their first manual with .308 Win. data used a Winchester 70 sporting rifle with a 22" barrel; if memory serves me right talking with their ballistics man who did it working there back in the '60's.
They've traditionally used rail guns testing for accuracy to eliminate all the rifle and human variables for quality control. People touring their plant often mistake them for pressure test barrels. The loads for each bullet listed as most accurate is proved so across several rifles.
Sierra has typically been conservative in their load data. They use the same thing most other data's derived from. Visual inspection of cartridge cases and primers and their opinion of what it shows. Here's their data from the 5th edition of their manual:
http://www.6mmbr.citymaker.com/f/Sierra308Win.pdf
It lists 43 grains of Varget as max under their 165. "Starting" load is 38.5. Current Sierra Infinity software lists that same rifle with 43 grains of Varget as max for their 165. Same software lists their 168-gr. HPMK maxed out with 43.5 grains of Varget and their 175 HPMK and all their 180's with 41.7 grains of Varget.
A given load will easily produce a 100 fps spread in average muzzle velocity across several barrels. That means its peak pressure is also not the same across all of them. And visual inspections of what shows excessive pressure varies across several humans. I've shot 7.62 NATO arsenal proof loads in military rifles whose fired cases and primers looked "a little warm but still OK" to several people. Go figure that out as they were spec'd at 67,500 cup and normal service load max spec was 50,000 cup.
But all that's someone's maximum load. Some use standards and others a wild guess. There are no standards for establishing a starting load. What some actually starts with may well be lower than what they publish as a starting load. Oft times, a 10% reduction of what the maximum load is used.
With all the .308 barrels having different internal dimensions plus the variables across all lots of powder and primers, it's only normal to see a wide range of starting and maximum load data.