The focusing effect is a cognitive bias that occurs when a person places too much importance or emphasis on a selected detail rather than considering the "larger picture" - this can lead to errors in predicting a future outcome. By emphasizing the importance of a very obvious detail sometimes the small factors (which can be just as important) are ignored making the actual result different from a predicted result.
For example, a famous study asked participants from California and the Midwest to rate how much more happy the California residents were than the Midwestern residents. Borth the Californians and the Midwesteners rated Californians to be more happy. But actually happiness ratings showed no significant difference between the happiness ratings of either place. The focusing effect is at play here - by over-emphasizing the obvious factors that might make California residents be more happy (great weather, things to do, etc.) the participants ignored the less salient factors that would decrease the happiness levels of Californians (cost of living, earthquakes, pollution, etc).