If you look at say 500 million shares traded in the Dow components over a period of 5-10 years, at various times of day, various market conditions, various quarters, etc... you will find that your chart will have price congestion at certain numbers...
Remember, trading stks is still just buying or selling, that's it. The culture of buying and selling is based as much on price perception as anything else. Next time you go to the grocery store see how many prices for goods are at .99, .89, .74 vs. .01 .00, .76. etc... The professional trader, the mutual fund holder, the trader in his kitchen and the wealthy hedge fund manager all have something in common...they all see these price pivots in the real world everyday.
This all important data, or bias is brought into the market, you therefore see a mirror of the same gravitation towards prices used for produce, dry goods, cars, houses, bentley's, toyota's and stocks.
-Ross