Authcache works occasionally, help

When I first request a page, the cache render is 'REQUEST FIRST PAGE', then refresh the page, the cache render is set to a number of milliseconds, and I know authcache works.

But If I request the same page in another tab or browser, the cache render is reset to 'REQUEST FIRST PAGE' again, in fact it should be some milliseconds as it had been cached.

What's the problem, am I missing something? thx in advance.