The problem is the query string on the end of the URI.
In this case ?70
Google PageSpeed does _hate this_ indeed.
I've looked at this. You can change the default behavior but it will require changing core Monstra files. It will be overwritten when you update, so it isn't a good idea. But...
Line 127 of engine/Plugin/Stylesheet.php is:
You can change it to:
and line 62:
$frontend_site_css_path = MINIFY . DS . 'frontend_site.minify.'.Option::get('styles_version').'.css';
You should probably System -> Settings -> Delete Temporary Files
Again, I'd advise against it.
You might be right to want to do it though.
http://www.stevesouders.com/blog/2008/0 … erystring/