In many fields, the first-listed author did the largest share of the actual work, and the last-listed author is often the senior researcher who supervised the project or secured the funding, without necessarily writing most of the manuscript. That convention is common enough to be treated as a default assumption — right up until it isn't, because a meaningful number of fields order authors alphabetically instead, with no seniority or contribution signal built into the order at all.

This is why a corresponding author is marked separately from the author list's ordering — that designation identifies who handles communication about the paper after publication, not who contributed the most. The two roles often, but not always, belong to the same person.

For readers moving between disciplines, the safest approach is to check the specific journal's or field's convention rather than assume the ordering means the same thing everywhere. Assuming a physics-style alphabetical order carries the same meaning as a biomedical-style contribution order is a small, easy-to-make mistake with a real chance of misattributing credit.