LanX,
For nearly a decade, I was in a torrid love affair with Perl and the Monastery but have not been active for the past couple of years. Here is what I think is going on:

A new node has a relatively small window of time (newest nodes - recently active threads) to amass the majority of its votes. If it does not reach a certain threshold, it is lost to the archives forever. That threshold continually increases because older nodes that were above that threshold and are still visible through Selected Best Nodes slowly accrue new upvotes dragging the "norm" up. Here is why I think this is what's going on:

When Selected Best Nodes was first introduced, the 50th randomly selected node had a reputation somewhere near the low 70's. Over time, I watched that number climb. This morning I saw that it is nearly 100. While I believe many extremely experienced monks have moved on from PerlMonks - there are still many quality nodes written that would likely have been in the best nodes if they had been written years ago.

For what it's worth, my memory of history is that Selected Best Nodes was created to fix this exact behavior because prior to that only the very top nodes could be seen.