When a bar report was built based on a database view, the count at the bar doesn't match with the count of records in list which comes up when we drill down the bar.
Any reported release.
Make sure you are not running into the known error KB0683833.
Then go to "Configure" tab in bar report ( in new UI) and check the "Aggregation" section. If the database view is based on different sys_class_name, but aggregation is done on bar with different class name, you will see different count in the bar than when you drill down the bar.
For example, lets say database view is based on problem, but bar count is set based on incident, then the count at the bar and count at the drill down of the bar doesn't match, because both are different task records.
Make sure the "Aggregation" configuration of bar report is configured in such a way that it reflects the same data which are shown when we open the drill down report from bar.