Via the XeTeX mailing list:

Dear all,

Here is my summary of WordSpace odds:

As far as I can see, WordSpace with single parameter sets all 3 fontdimens
to the same value (and do not scale them to the same ratio, as the manual
states). In the second invocation it uses the previously set values, not
the default ones (this is probably not a bug) and scales everything again,
UNLESS the new ratio equals the previous one (examples 3.1 and 3.2). The
bad thing is, that if used stand-alone (example 3 and 4) the changes spread
across the enclosing group boundary.

Yours,
Vadim

(fontspec.sty v1.18)

\documentclass{minimal}
\usepackage{fontspec}
\setmainfont{Garamond Premier Pro}

\def\test{Space=\the\fontdimen2\font+\the\fontdimen3\font\the\fontdimen4\font/\the\spaceskip}

\begin{document}
1. {This is a test. \par \test}

2. {\addfontfeature{WordSpace={1,1,1}}This is a test. \par \test} \par \test

3. {\addfontfeature{WordSpace={2}}This is a test. \par \test} \par \test

3.1. {\addfontfeature{WordSpace={2}}This is a test. \par \test} \par \test

3.2. {\addfontfeature{WordSpace={2.1}}This is a test. \par \test} \par \test

4. {\addfontfeature{LetterSpace=2,WordSpace=2.5}This is a test. \par \test}
\par \test
\end{document}