Moe Green estimates the cost of future projects for a large contracting firm. Mr. Green uses precisely the same techniques to estimate the costs of every potential job, and formulates bids by adding a standard profit markup. For some companies to whom the firm offers its services, no competitors exist, so they are almost certain to get them as clients. For these jobs, Mr. Green finds that his cost estimates are right, on average. For jobs where competitors are also vying for the business, Mr. Green finds that they almost always end up costing more than he estimates. Why does this occur?