Does anyone know how the "shuffle" feature on iPods works?

It amazes me how much music there is on my little teeny 16 gigabyte thing about the size of a large postage stamp - something over 3000 tracks. I don't even think about how they manage that any more, but the algorithms behind the shuffle logic seem really weird to me. You'd think (I'd think anyway) that with the same shuffle list going I should be able to not hear the same song twice in weeks worth of commuting (at about 90 minutes a day on the road). The thing seems to me to go in weird cycles though - lately it's been trying to convert me to some of the more obscure early tracks from Stan Rogers - when he was a coffee house folkie, and hadn't yet become the World's Champion Canadian. Some of the tracks are ok, but mostly I want to hear Barrett's Privateers and Northwest Passage and Mary Ellen Carter, etc. but it's tried to make me listen to "Picture of the Past" (or something like that) about sixteen times in the last four days.

 And Ramblin' Jack Elliott - now it's true that I have quite a number of RJ's tracks (mybe 40 or 50) but over the past couple of days every other damn track is Ramblin' Jack, and a lot of it's just not that entertaining. Why is my iPod trying to make me memorize everything The Carter Family ever recorded?

On the upside, the iPod seems to like Django Reinhardt & Stephane Grapelli as much as I do, so I can't complain there. If anyone really knows how that feature works I'd be interested to hear it.