Computed items and data functions are fundamentally different, and the functions available in the Computed Item dialog box do not calculate data in the same way as data functions.
Computed items calculate a fresh value for each original value, based on the computation (for example, Revenue calculated from Price and Units Sold). The new values are part of a new data item or replace the original values. Computed items never reduce the original number of records.
Data functions, by contrast, summarize groups of database records and replace the original values with new summary data. Because data functions summarize values, the number of records are frequently reduced.