currently the version 1.9.32.14-r0 of mod_pagespeed running on the webserver
purely static content (not pagespeed cached) is served with first max-age=300 and counting downward to max-age=0 and starting again with max-age=300
when mod_pagespeed is disabled then mod_expires/mod_headers continuously displays the .htaccess configured max-age=2692000
i don't see the reason why
.htaccess:
<IfModule mod_expires.c>
ExpiresActive On
ExpiresByType image/jpeg "access plus 2692000 seconds"
</IfModule>
<IfModule mod_headers.c>
<FilesMatch "\\.jpg$">
Header set Cache-Control "max-age=2692000, public"
</FilesMatch>
</IfModule>