Why the ETAG always change of my D7 site ETAG should not change if the page not changed.

But when I refresh the web page after login, the ETAG always change.
In my setup, if I am logged into the CMS, then the Etag seems to be written as a unix timestamp, which is clearly being done to force caching off.  So this may be the reason you see the Etag constantly changing.

However, even when not logged in, in my case the Etag seems to change when the associated page cache expires, even if the content in the resource doesn't change at all.  I don't know why this is happening.