Even the definition of "economics" varies according to which economist answers the question, so economists naturally disagree on finer points of the complex field. Economics is based on psychology, statistics, projections of future innovation and projections of the future of political events, which leads to vastly differing projections from economists.
Many definitions of economics treat it as a social science, but economists often work in fields unrelated to the more scientific and theoretical aspects of the field. For example, many economists work as advisers for investment companies. Since stock markets and other fields are inherently unpredictable according to many experts, different economists are likely to have different interpretations of how to predict future performance. Because the field is so open to interpretation, economists are likely to find data that support an array of differing opinions.
A major factor in economics is the nature of demand, which experts study using psychology, another field fraught with disagreement among experts. The field is often intimately tied to politics; Marxism, for example, is largely a theory of economics and how it relates to members of a society. Libertarian theory attempts to refute many Marxist claims. Since economists come from an array of political backgrounds, many disagree on how to interpret data and studies that purport to back up or refute particular ideas.