"Official" journal abbreviation of MAGMA
There is probably nobody but the particular administrators of the various organizations involved who can speak to the exact details of why this journal's abbreviations are inconsistent.

I suspect, however, that the answer that you would get, if you could track them down would be something along the lines of:


  I don't think it's inconsistent... hmm, I guess maybe it is?  We should probably try to figure out how to get that sorted out at some point.


The point is that curation is hard, curation of periodicals is harder, and curation of periodicals over long time frames and across multiple organizations is particularly difficult and easy to get wrong.  Worse yet, repairing a problem will often create more problems for the future (e.g., an inconsistency might get fixed by hard-coding in an alias).

Even within a single organization, it's difficult to handle different parts of a process consistently: for instance, yesterday I proofed my final version of a paper for a major journal and found that some of the links to instructions that the production staff supplied were incorrect.

All of this is a long way of saying: probably it's just an unintentional mess.  You can point it out to the people involved, and they might be able to fix it; they might also have work-arounds in place already and not care.  At the end of the day, however, it need not be your problem, unless you are in charge of curation of the journal or other resources involved.