High distinctiveness led David's boss to this conclusion.
When a person behaves uniquely in certain situations it refers to the distinctiveness. There are two measures for this such as high and low distinctiveness. When an individual is behaving similarly in all kinds of situations it refers to the low distinctiveness. When a individual exhibits only one behaviour in a particular situation it refers to high distinctiveness.
In the given scenario, David’s boss observes that the time taken by David in completing the tasks A,B,C is about 12 hours. He took 24 hours to complete the task D. Thus, the boss of David concluded the task D is tougher than the other three because of high distinctiveness of David’s behaviour. David shows one particular behaviour of delayed working of task D in that particular situation.