OVERKILL
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I wrote on this in a previous exchange, but some of the approvals are a double-edged sword. This fosters innovation, in some respects, as oil companies have to work to get around the limits imposed, but sometimes those limits aren't overly applicable to a given situation.
Take our recent discussions about full-SAPS oils being "obsolete" because of the various ESP formulas we see. Porsche A40, LL-01, MB 229.5, VW 502/505...etc are all approvals that applied to this style of lubricant, which had higher levels of AW additives than where the API put us on this side of the pond. The main driver for this was catalyst protection, but I don't recall full-SAPS oils actually resulting in failed cats in service, so...
Some of these oils had some pretty wild pedigree. M1 0w-40 was used in the 24hrs of LeMans for example by numerous teams including Porsche, and by the GM Corvette race program. It was also used in the AMG race program. That's pretty bloody impressive for an OTS lubricant.
Now, the Euro marques are big into the ESP oils and GPF's, which are more sensitive, have been introduced, and that's one of the drivers behind these formulas. The same on the diesel front with DPF's. As emissions systems become more complex, protecting them from oil constituents, in the event somebody has a "burner" has come to the fore. This limits certain types of additives and concentrations and this also has a reverberation affect, as it ties into LSPI, where some of these components that have been reduced, were effective at mitigating LSPI, so then calcium needs to be scaled back to compensate, otherwise you end up with a formula that may promote LSPI, and then it gets replaced by something else that is perhaps less effective.
So, you have this clutch of compromises that have to be made to comply with regulation, and this regulation may be at odds with some of the other goals of your product, such as wear protection. This is where alternative compounds come into play, to aide the formula in achieving acceptable performance in those areas while still meeting the criteria to comply with regulation.
An example of where the goals of the product clash with the rules of the API, would be with the reduction in allowable levels of phosphorous, which limited ZDDP. We saw oils that this limit applied to, like Castrol GC 0w-30, stick with API SL, rather than going up to SM. There were other OE products that completely avoided or abandoned the API approvals altogether, like the BMW 5w-30 for example. For the Euro lubes, the API approvals aren't important anyway, as ACEA is what they use as their foundational platform, so it wasn't a big deal. Blenders dealing with the North American market however, didn't have that flexibility on many of the grades.
So, as @RDY4WAR noted, while the API does indeed set a floor for the oils sold for the North American market (and that's a good thing!) it also sets a ceiling in many respects, because the goal isn't just product performance in terms of what's mandated for protection, durability...etc, but also limits are placed on components to ensure emissions systems protection under typical operation, which may include significant oil consumption.
Of course, oil manufacturers are also blending to a price point and to make money and two oils that both have the same API approval, well, that's a bar set pretty low, relatively speaking, in many respects. One oil can perform markedly better than another there. That's why the OE approvals, particularly the Euro ones, raise the bar by capping Noack and mandating a level of performance in demanding engine tests.
The old Mobil slides show this quite well:
And even relative to their own products:
