I've seen many cases like this.
One issue is the grade into which an oil falls is rather wide. You can test 3 oils and one will be built in the low end of the grade, one might be in the middle and the last one on the high side of the grade.
Also, I forget the exact testing procedure, but the oil being tested needs to be "in grade" for a certain number of tests. So let's say out of ten tests on any particular oil, if that oil passed the grade test 5 times out of 10 it's considered in grade.
If an oil is built on the low side of its grade, and it starts getting some load on it, like at the thrust or exhaust valves or whatever, it will get hot enough that it falls out of grade. And then the oil can't get rid of the heat that's pushing it out of grade, so the parts get hotter and the cycle continues.
It's almost impossible to get all the information from the oil manufacturers. The only real way to know if an oil is capable is to test it. And that costs big $$$$$. And it takes time.
Or, you learn the way you did, which isn't cheap either.
