With xelatex both values for 1ex are the same, but with lualatex I get (10.67352pt) versus (3.43344pt) The reason why the sans serif \LaTeX is not correct. This happens only for LinBiolinum_R.otf and I do not know why.
\documentclass{article}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
\setsansfont{Linux Biolinum O}
\newdimen\EX
\begin{document}
\Huge
\LaTeX (\EX=1ex \the\EX)
\sffamily
\LaTeX (\EX=1ex \the\EX)
\end{document}
Herbert
