browser caching strategy for javascript and css?

i have found that my site's js and css files are not being cached in browsers correctly. when i use pingdom tools analysis and other similar tools, i was get a warning about 'leveraging browser cache' - highlighting that the core and plugin css/js files are not being cached by the browsers. when i add a directive to my site's nginx config file that is intended to set the expires header for jss and css files, it appears to do nothing - even though i have successfully added such headers for other file types already.

is there something about the way that elgg serves js and css files that prevents an expires header being added!? when i analyse the elgg community here through the same tools, it passes 100% and gets no warnings.. anyone know how/why?