An expert makes decisions based on everything that he knows. As you can probably imagine becoming an expert is very difficult. Computers also have trouble weighing all the different types of analysis and coming up with a composite decision. Experts can weigh all the evidence, come up with a plan with contingencies if things don't go right. This is very difficult for computers.
Experts can change their methods and strategies.
Computers are stuck with their strategies once they are programmed and released. Most experts utilize mechanical timing methods to help them make decisions. They will then sometimes override the mechanical methods based on knowledge that can't be represented with computer modeling. This is a very important concept. Mechanical timing methods operate on stock market closing data.