Moves that don't conform to known patterns are often ignored. The opening is another crucial area of difference. Centuries of chess playing have honed the opening and a large body of theory has developed. Chess players study as much theory as they can and spend years understanding its nuances. At the opening point of a game, the number of possible moves is so high that mere calculating ability and brute force of the computers are mostly rendered useless.
Humans are also highly inconsistent creatures.
They can be affected by breakfast, bad sleep, fear, nervousness, while computers are impervious to everything but the electricity. Garry Kasparov is famous for making condescending remarks in order to psychologically intimidate his opponent. Deep blue tipping the scales at 1.4 tonnes to Garry's 176 lbs probably wouldn't be intimidated. Technology eventually brought out the good of the machines. In 1986, the first chess database was born.