We humans are never particularly good at understanding the concept of "random". Computers, while not perfect, do provide a pretty good approximation.
For a thought experiment, imagine the odds of rolling a 3 on a single 6-sided die. 1 in 6, right? So on average, if you roll a die every day, you would expect to see each number about every six days. Here's the thing though... The odds of it actually happening that way, to choose each of the 6 numbers without a repeat, is only 1.5%. So there will be at least 1 repeat, 98.5% of the time. (1st roll can be anything (6/6), 2nd roll can be 5/6, 3rd roll is 4/6, then 3/6, 2/6, and finally the 6th roll it needs to be 1/6). Multiple those together, and there's the 1.5%....
Same phenomenon here, folks. There just aren't as many users as we might think that meet the selection criteria. Chosen randomly from that criteria, and there are always going to be repeats. Sometimes it's going to seem like there are many of them.
But people do convince themselves that there are patterns, which is why casinos put those electronic displays next to roulette wheels. ("It just came up red 4 times, it's obviously going to be black this time!") Apple users got their noses all bent out of shape in the earlier days of random shuffle on ipods, convinced that the programmers hadn't programmed "random" correctly, after hearing more repeats than some would expect. To fix the problem, Apple had to put in quite a bit of code to make the random shuffling *not* random, and instead weight it much more so to not repeat songs truly randomly. We are peculiar creatures.
