GameObjects appear with an offset

Hello there,

I am having a strange issue with Hololens. I place my game objects in 10cm in z-direction of the image target and they look correct. When I increase the distance 30cm the gameobject's position has an offset (the center of the gameobject is not aligned with the real world postion). I tested it with a ruler on the surface of a desk. I am using Vufroia 7.5.26 with Track Device Pose enabled and Tracking mode on Postional and Fusion mode optimize for image targets. I am working with Unity. Any ideas what causes the offset?

Best regards

-Dana Scully