For 12 gauge 70mm, 2 3/4" the SAAMI pressure level is 11,500
For 20 gauge 70MM, 2 3/4" the SAAMI pressure level is 12,000

You are mixing 12 and 20 levels. 20 gauge pressures run higher than 12 as do the 28's. And 12,000 is top end but still in the good range. Problem is they also run plus or minus on pressures. I have seen them run 1,000 plus psi with almost no difference in velocity.

This is a link to a good explanation and better chart.
https://randywakeman.com/shotgun_pressures.htm

You can not infer that American shells pressures are anything because we just don't get that information. Shell length does vary from maker to maker and lot to lot. I am in the middle of trimming down a lot of hulls, loading them for pressure and velocity testing to get a long term option for all my classic doubles. If things work as well as I expect I will start out with 5,000 shells loaded in five basic configurations for all my basic shooting needs. Then I will buy another 15,000-20,000 hulls to take care of those needs for the rest of my shooting lifetime. No more concerns about what is safe, what is available or what powders can I substitute for impossible to find powders.