Troubleshoot EPiServer extensions for the output cache
Expire time in the web page header is set to -1 day
-1 day is EPiServers default value for the expiration and will be set if
1) The user is authenticated
2) Http cache time out set to zero
3) Http method is not GET
4) If the fetched page is a version
References
Description of how the EPiServer extensions for the output cache works