Cache CSS and JS files

When I refresh my website in less than 2-3 minutes, Firebug shows these nice requests:

1. /core.css          304 Not modified
2. /core.js           304 Not modified
3. /background.jpg    304 Not modified

BUT when I refresh after >3 minutes, I get:

1. /core.css          200 OK
2. /core.js           200 OK
3. /background.jpg    304 Not modified

Why my CSS and JS files are downloaded again and images aren't?

I'm using ASP.NET MVC 3, I DON'T use [OutputCache], and in my /Content folder (where all css, js and img files live in subfolders) I have this Web.config:

<configuration>
    <system.webServer>
        <staticContent>
            <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="1.00:00:00" />
        </staticContent>
    </system.webServer>
</configuration>

which sets HTTP header Cache-Control: max-age=86400 ONLY. So basically CSS, JS and images are treated the same way, but somehow CSS and JS don't get cached for a longer period... why is that?