I can't see your particular environment being the issue.
I can reproduce what you said and am on Debian 7.1 (wheezy) and Iceweasel 17.0.9.
It's likely that there are two different scripts, one when you load the page manually by clicking the "answers" link and then "newest", and another script which auto reloads the page.
Both cases I took the HTML and CSS and returned the same (barring the text h vs hours):
<span class="relativetime" title="2013-09-28 18:49:27Z">7h ago</span>
<span class="relativetime" title="2013-09-28 18:49:27Z">7 hours ago</span>
So possibly two different scripts, both use the same template but the two back end scripts pass a slightly different time to display to the template.
This is all speculation, but this can often happen in a large setup. ie functions get bolted on later, or a different developer, etc.