A house painter has a regular contract to work for a builder. On these jobs, her cost estimates are generally right: sometimes a little high, sometimes a little low, but correct on average. When her...

A house painter has a regular contract to work for a builder. On these jobs, her cost estimates are generally right: sometimes a little high, sometimes a little low, but correct on average. When her regular work is slack, she bids competitively for other jobs. “Those are different,” she says. “They almost always end up costing more than I estimate.” If we assume that her estimating skills do not differ between the two types of jobs, what can explain the difference?