I'll try to demonstrate my issue with a simple example.
Suppose I compile the following file (I used LuaLaTeX, TeX Live 2015)
\documentclass{standalone}
\usepackage{tikz,pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
xlabel=$x$,
ylabel=$y$
]
\addplot[line width=0.8pt]{x};
\end{axis}
\end{tikzpicture}
\end{document}
If I examine the produced pdf file with a vector graphic program (e.g. Adobe Illustrator), I note that the line width of the graph is 0.797pt instead of the requested 0.8pt.
Although I know that the error is negligible for all practical purposes, it would be interesting to know its cause.
ptin TeX) = 0.99628 big (PostScript/Adobe) point (bpin TeX). See also http://tex.stackexchange.com/a/8337/21344 – Paul Gessler Aug 20 '16 at 15:07