AGSM bibliography style sometimes doesn't abbreviate to "et al." for duplicate author+year

You've come across an unusual -- and admittedly rather severely under-documented -- feature (not a bug...) of the agsm bibliography style. Suppose two bib items labelled, say, AA and BB each have one or more authors. Crucially, suppose the total number of authors differs -- e.g., let bibitem AA have 3 authors and bibitem BB have 5 authors -- and suppose further that AA and BB share the same first author (say, XYZ) and the same publication year (say, 2000).

When this occurs -- as is the case in the example you've posted -- the agsm bibliography style does not set the citation call-outs as XYZ et al (2000a) and XYZ et al. (2000b), respectively. Instead, it lists all author names for both publications.

I suppose this is a fail-safe way of avoiding any kind of confusion over whose publication might be cited as XYZ et al. (2000a).

The only time when you do get the FirstAuthor et al (year) citation call-out pattern is if (a) the two publications have the same authors (and thus the same number of authors) as well as the same publication year and (b) there is no other three-or-more-author publication in the bibliography that features the same first author and publication year.

Again, this feature of the agsm style is both uncommon (to put it neutrally) and, unfortunately, quite obscure and under-documented. I wouldn't call it a bug, though. If you truly can't stand this feature, it's probably a good idea to start looking for an alternative bibliography style.

An MWE and screenshot:

\RequirePackage{filecontents}
\begin{filecontents}{testagsm.bib}
@article{1,
  author = "XYZ", title = "x", journal= "y", year = 2000}
@article{2,
  author = "XYZ and B", title = "x", journal= "y", year = 2000}
@article{3a,
  author = "XYZ and B and C", title = "x1", journal = "y", year = 2000}
@article{3b,
  author = "XYZ and B and C", title = "x2", journal = "y", year = 2000}
@article{4,
  author = "XYZ and BB and CC and DD", title = "x", journal= "y", year = 2000}
@article{5,
  author = "XYZ and BBB and CCC and DDD and EEE", title = "x", journal= "y", year = 2000}
@article{x1,
  author = "A and B and C", title = "D1", journal = "E1", 
  year = 3001, volume = 1, number = 2, pages = "3-4"}
@article{x2,
  author = "A and B and C", title = "D2", journal = "E2", 
  year = 3001, volume = 5, number = 6, pages = "7-8"}
\end{filecontents}

\documentclass{article}
\usepackage{natbib,har2nat}
\bibliographystyle{agsm}
\setlength\parindent{0pt} % just for this example

\begin{document}
\begingroup
\obeylines
\citet{1}
\citet{2}
\citet{3a}, \citet{3b}
\citet{4}
\citet{5}

\medskip
\citet{x1}, \citet{x2}
\endgroup

\bibliography{testagsm}
\end{document}