Loading Atahualpa css/js from browser cache

I want to start using expires and cache-control headers to improve the load time of my site after the user first loads the home page. While doing some tests using Firebug, I noticed one odd thing. When I have a primed cache, all of my external JS and CSS requests return a status of 304 Not Modified, except for the Atahualpa ones:

http://expat.cl/?bfa_ata_file=css
http://expat.cl/?bfa_ata_file=js

Which return 200 OK. The Last-Modified date updates with every page load.

How can I fix this? (And I guess I should ask, am I correct in my understanding that the reason for choosing to externalize JS and CSS is so that they can be loaded from the browser cache on subsequent requests?)

Thanks!