Due to Salesforce limitations on the Activity object (tasks, events, and emails are all versions of the same Activity Object) you can not filter down to a subset of tasks when filtering from a parent object in an Apsona report. In other words, you can not filter tasks with a 'quantified filter'. The report will return all related tasks, not just the ones identified in the filter.
The way around this limitation is to use our Multistep reporting tool. You can create a step on the parent object, applying any necessary filters; and then a second step on the Task object where you can further filter directly on that object, and thus reduce the results to just the task records you need.
NOTE: Due to other Salesforce limitations, you must use the Task object labeled with the parent name in order to properly link the two steps together. For example, if you are wanting a subset of tasks related to Opportunities, you must associate your second step to "Opportunity Task", not just "Task". If you want to find Account related tasks, you must associate your second step to "Account Tasks".
For more information see this article.