Willi,
A few baselines:
1) Many MFGs are famous on this site for not replacing their reamers or other tooling until tolerances are visibly out of whack. We're aware of that problem, and it's real.
2) Sako/Tikka have been holding a thou since before it was cool. When they don't, it's a notable outlier that's worth posting about. Whereas, if a Remington was within 2 thou of its spec that would be a notable outlier. It's not that every Tikka action is perfect, it's that the vast majority are, and that's enough to support 5 or 6 good pre-fit businesses, plus McGowen. These companies generally spin shouldered pre-fits for Tikkas and guaranteed customs. Not factory actions which don't generally hold a thou. Of course they also do barrel nut designs.
3) Some customs are advertised as holding a thou or less, some aren't. Will you get tighter headspace in all cases by spinning to the host action? Yes. Does it matter? That depends. Sometimes for some applications. Usually not.
Glad you joined the site, don't get intimidated by the reception - there is a common base of understood knowledge in this group that you're peaking into now.
Best,
-J