By default an Arial clone is present, ideally Arial is specified as a valid replacement font in the PDF if the user does not have the Arial clone present (Arial itself is a clone, but that is another story).
It would require deeper investigation to see if this is actually the case. I've always wondered about this.
PDF/A has given me all kinds of issues (windows users get incorrect glyp placement with very bad results). Regular PDF has worked fine for me.
nit: normal PDFs may embed fonts, but PDF/A must embed them, and PDF/A is not the default in most programs.
In practice however most programs seem to include fonts in exported PDFs?
Linux machines don't normally include Arial due to the license, and only PDF/A includes the fonts used in the document.