[Plugin: WP Super Cache] Issues With Gzipping Content

I am using the Wp-Super Cache plugin on my blog with Super Cache Compression enabled.
I used http://www.whatsmyip.org/http_compression/ to test if my site is Gzipped or not. It states that the home page is gzipped, but others are not.
Example

Why is this happening?

http://ww.wp.xz.cn/extend/plugins/wp-super-cache/