Cached files going away

I’ve set the cache to expire in 24 hours in advanced-cache.php. I reload a page a few times and see the 4 commented lines and know it is working.

I come back an hour or two later and see the following comments meaning the cached file from hours ago is gone and a new one was just generated 116 seconds ago.

Why isn’t the first cached file still around?

<!--
	generated 116 seconds ago
	generated in 0.260 seconds
	served from batcache in 0.000 seconds
	expires in 86284 seconds
-->

http://wordpress.org/plugins/batcache/