[XeTeX] Why XeTeX runs are non-deterministic?

[Paulo Ney de Souza]

If you take a simple latex file like:

\documentclass{article}\usepackage{lipsum}\usepackage{amsgen}\begin{document}\lipsum[1-10]\end{document}

and run latex, pdflatex or lualatex repated times (on the same file) you
get the SAME dvi or pdf file up to an internal DATE and ID in the file --
and everything else is the same.

No so with "xelatex" where different runs on the same file produces PDF
files of different sizes and whose internals differ by quite a bit. To see
the differences you can use "vimdiff file1.pdf file2.pdf".

Why is it so? And what generates this difference? The question has been
lingering on SX for a while (
http://tex.stackexchange.com/questions/292675/why-does-xelatex-produce-different-files-from-the-same-deterministic-sources)
but with no reasonable answer!

Paulo Ney de Souza
pauloney at gmail.com
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