The url is linked to the last post on the page. When the page loads it will go to the post, but when (uspecified siezed) images from upper posts are downloaded and displayed the location of the page doesn't stay at the linked post, but it moves upwards.
While not exactly a duplicate of bug 3390, this behaviour is described in comment 21 of bug 3390. It would most likely be addressed by a fix to that bug.
<http://bugs.webkit.org/show_bug.cgi?id=3390#c21> is a direct link to the comment I mentioned.