Here's the full request (less api key etc.):
http://developer.echonest.com/api/v4/playlist/static?_=1384961698349&artist=yes&artist=genesis&artist=pink%20floyd&artist=king%20crimson&artist=emerson%20lake%20and%20palmer&description=art%20rock&description=progressive%20rock&description=british%20rock&type=artist-radio&variety=1&distribution=wandering&artist_start_year_after=1971&artist_end_year_before=1989&bucket=id%3Ardio-US&api_key=XXXX
What I'd expect is some variety. But the no matter how often I load this (with changing _ parameters, of course), I would always get very similar to almost identical results. As described in my first posting, the first three tracks would be the same three artists with two of their popular tracks. That much variety I did get, but not much more.
I tested this with two different browsers and programmatically. With various buckets. While the tracks do fit my criteria, I would have expected more variety. But the results were repeating, over and over again.