Tick a task on your Tasks page and it is crossed off and sinks to the bottom of My tasks. The last 10 crossed-off tasks stay there so you can see what you got done; anything older is quietly cleared. Tap the box on a crossed-off task to bring it back as an open task.
Tasks you finished inside a session are counted on that session's recap ("N finished") - they are that session's history and do not pile up on the Tasks page.