On Ubuntu 16.04, Unity -- and may be earlier Unity -- there's a height of a top system toolbar added to a location point of a common print dialog if it is launched via ServiceUI.printDialog(), be it with a given screen config or null (primary config).

A small test attached.
On i.e. Ubuntu 14.04 with Xfce it puts dialog to 100,100, and on Ubuntu 16.04 with Unity to 100,128 (or 100,156 on a HiDPI monitor).