When I'm filter on (for example) priority 3, why is a task which I add not added with priority 3, but 4?
Please clarify - you're choosing a filter that should show tasks with the third priority and among them, you see one more task (that's not a sub-task) which has no priority (default priority 4)?
If so, please try to edit this task and check if it really has no priority set and save it and see if this helps.
When I going to filter and press the + sign, the default priority is 4 (which is displayed in the task) and not the one I used in the filter.
After saving it's saved under the prio 4 and not under prio 3.
Of course the prio can be changed, but when you are in that filter assumption can be make that take added will take over the filter setting
Thank you for clarifying. Unfortunately, the content of the filter does not affect the default settings or properties of a newly added task. We'll consider implementing this in the future (if possible) and you're welcome to add a request for such change to our Votebox: http://todoist.com/Vote_iOS/