Random song
Random song
Collective Soul song
Collective Soul song
Dave Matthews song
Dave Matthews song
Random song
Tom Petty song
Tom Petty song
Random song
Random song
Random song
...

This really does happen quite a bit

Yup, and if you look at the output of FerretBoy's program, you can see why this is normal and common in a truly random shuffle.

It's all about perception. I was (as you are) perceiving that this is unusual because you're sitting there, in "the moment", hearing two Tom Petty songs in a row. Or three in a group of ten, or whatever. And you're thinking to yourself that's gotta be a mistake. Because all you're hearing is "that moment", that short period of, say, ten songs, while you drive to work. So any statistical clustering is going to sound like a mistake. But what you're not seeing (and what FerretBoy's program clearly shows) is where all the other Tom Petty songs are in the shuffle. And how they releate to all the other songs on the player. And once you see it in that light, suddenly some statistical clustering makes perfect sense in a random sample of that size with the artist/album criteria which makes you consider something a cluster.

See, in order to hear and perceive the true distribution of an artist or an album in the shuffle, you have to listen to the whole shuffle. And with thousands of songs, that would take days. No one ever listens to their player for days at one sitting. You listen to it in short groups, as you're driving, or working. And your perception is colored by that sub-segment of the larger statistical sample.