Nozzle offset
If parts placed with a specific nozzle have a consistent offset in the X and/or Y axis, there are a couple potential root causes:
The nozzle offset for that nozzle needs adjustment
If OpenPnP doesn't have an accurate idea of how far away a nozzle is from the top camera, it's going to place them with that inaccuracy. Adjust that nozzle's offset.
The bottom vision pipeline for your parts needs adjustment
OpenPnP decides how to place parts onto your board based on what it sees in the bottom camera view. If a bright light or reflective surface in view of the camera is making OpenPnP think your part is a different size than it is, you'll see a placement offset. Adjust your part identification pipeline.
The nozzle's tip calibration needs adjustment
When OpenPnP performs a nozzle tip calibration, it profiles any runout in the nozzle tip. If this calibration does not correctly identify the nozzle tip, it can cause placement errors. Adjust your nozzle tip calibration pipeline.