When cropping an image with a non-box bounding object (e.g. cylinder), the outside value is always set to 0 (which looks gray in images with signed voxels), instead of the value chosen in "Set border voxel value".
To replicate, load a a signed image (e.g. Pic3D), activate "Set border voxel value", choose a value different from 0, choose a bounding volume different from box, and crop.