It's just a different opinion of what is standard.
Good question. Have you asked in their forums?
If you follow the link provided by user poster, you will see that MM maps certain other fields to year. And perhaps they don't want to loose any data and create a second YEAR field ...
As MP3tag accurately displays even the second (or more) YEAR field, I do not see any compatibility problems. I do see implementation problems on the side of the other programs.
What I don't understand: you ran into tagging problems because you reached the memory limit and it is still not possible to think of a different workflow?
Did you know that it is possible to d&d files from iTunes directly into MP3tag for editing? With (just as an example) iTunes as the administration program for the whole collection you could pinpoint exactly those files that are necessary to perform your current task. So you might save the MP3tag features and still access your whole collection.
Haven't asked in their forums as I'm new to MM and just noticed the issue when I retested Mp3tag and Audacity is not a primarily a tag editor. It would appear to me that MM and Audacity see the same year field, with Mp3tag being the odd one out, tho perhaps if we considered more programs, the majority vote would come out differently.
I use iTunes for adding lyrics, but I don't like its interface and options for other tag edits. My workflow is the dog here, not the tail.
Again, my recent comments are designed to suggest improvements to Mp3tag, as I hope to come back to it in the future. We can always learn from how others do things.