Caching doesn't seem to be working

We have a site recently converted to WP (http://www.babypips.com/), with WP Super Cache installed with the following settings:

Caching – checked
– use PHP to serve cached files – selected

Miscellaneous – Compress pages – checked
– 304 Not modified browser caching – checked
– Don’t cache pages for known users – checked
– Cache rebuild – checked

Advanced – no checks

The Cache Tester says the pages Do Not Match, but I figure that is because I have don’t cache pages for known users checked (I have to be logged in to run the test.)

So I tried checking the timestamps from 2 other incognito browser windows in a different browser than I’m logged in as, and they always appear to be generating a new page every time I check–even though I shouldn’t be a “Known User”. I even tried logging out in the other browser, in case WPSC was using IP address to determine if I’m a known user.

Here are the timestamp comment sections from the two different incognito windows:

<!-- Dynamic page generated in 2.233 seconds. -->
<!-- Cached page generated by WP-Super-Cache on 2013-10-07 10:17:58 -->
<!-- Compression = gzip -->

and

<!-- Dynamic page generated in 2.451 seconds. -->
<!-- Cached page generated by WP-Super-Cache on 2013-10-07 10:18:21 -->
<!-- Compression = gzip -->

I read this as WPSC is letting WP re-generate the dynamic file each time.

Am I wrong, or is this setup not working correctly?

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