They are all a standard to and of themselves. To think otherwise would be to say that API, ASME, AISC, AWS, Ford, Chrysler, John Deere, NASA, etc all have to use the exact same terminology and definitions of said terminology.
Just because D1.1, D1.5, and A3.0 fall under the bounds of AWS does not mean that they are in perfect alignment. And especially when they are reviewed and edited and updated at separate times by different people from different industries and backgrounds. One minute they may align and then somebody changes something and it takes a while before it is brought into alignment with all the other resources.
He Is In Control, Have a Great Day, Brent