Ask people for a random number between one and ten and seven comes up far more often than one time in ten. You can almost hear the reasoning. One and ten are the edges, too obvious. Five is the middle. The even numbers feel tidy, three feels too early, nine too near the end. Seven is what is left once you have thrown out everything that looks chosen, which is how the number that feels least picked became the one picked most. Being random is hard to do on purpose, because every way of trying leaves fingerprints.