Card players sometimes get lazy, shuffling a deck fewer times than necessary to randomize the cards. Indeed, persistently sloppy shuffling can have a significant impact on play–an effect that experts (and gamblers) can exploit to their advantage.
However, the problem lay not in the computer but with human expectations. Subsequent research showed that hands in which suits are evenly distributed, that is, hands with four cards of one suit and three of each of the other suits, are actually more common in games in which people do the shuffling than they should be according to theory.
The reason for such a pattern is that cards during bridge play tend to clump together in groups of four of the same suit. Careless shuffling doesn't do enough to break up the groups, and when the cards are dealt out, the four players get a fairly even distribution of suits.