There might be the different solutions to this issue:
1. If the user has selected the wrong data source and the data you are getting is wrong. check data source selected is correct or not.
2. If the table or sp or view used in the dataset is different. Sometimes, while creating a dataset users write different table/SP/view names by mistake. this also leads the user to get the wrong data in the dataset preview.
3. Dataset selected for that widget is incorrect. This might happen because in some cases user creates 2 datasets with the same name but in different folders. one is for development purposes and the second is for live servers. it may happen that the user selects a dataset from a different folder and hens getting wrong results. it is advisable to select a folder before selecting a dataset from the dataset menu of widgets.
4. Data returning from SP or view is incorrect. For the complex calculations requirements, users prefer to create views or stored procedures. In these stored procedures and views, data is fetched from multiple tables and returned to one result table. It may happen that after execution of view or SP result table holds wrong data and same is reflected in dataset preview. you can diagnose this error by storing results of SP or views into one temporary table and checking requesting data from dataset preview and you will find the differences.