I'm guessing that during the time you spent reading nodes on the first page, three new ones were added: if your browser caches reasonably, you might not notice this (if, for instance, you use the "back" button instead of clicking on the go to Seekers of Perl Wisdom link at the bottom of the page). However, when you ask for the next page, you're getting it fresh and hot out of the oven, so it gives you the 11-20th nodes counting back from the moment you clicked the link, not the ten after the ones on the current page.

This might be construed as an interface issue, though I don't think it qualifies as a "bug" per se.