If you delete a task from a process, but had prior instances of the process that included that deleted task, it'll still show because that was part of the history of that process.  There was likely field values and other changes submitted with the task, that are still part of that history.

This is similar to how Workflow keeps revision history, and is part of the reason, if you've ever examined the LFForms database, why a lot of the configuration tables on the database have a deleted flag instead of just being deleted out of the database, for the sake of that history.

Short of deleting the actual instances, the history remains.