That’s only true in some areas (e.g. some biological fields). In other areas (e.g astronomy or much of physics), it’s decreasing order of contribution. In mathematics, the author list is usually alphabetical.

It is also traditional that the first author, in addition to doing the most of the scientific work, takes on most of the writing He/She also edits together the various other authors contributions.

One metric used to evaluate publication history is the combined count of first and last authorships for this very reason.