The thing is that yes, they must meet a MINIMUM set by the API, but that doesn't mean they can't be better than the minimum.
All 5 weights are NOT the same at -30C. They all are at or less than the maximum allowed by the API, but they are NOT the same. Going through some data sheets will point that out in a hurry!
And some oils call themselves a higher w rating than the really qualify for, simply because some buyers a leery of buying an oil that is "thin" based on that number - One of the Schaeffers 15w40 oils is the one that comes to mind, if I am remembering correctly.