Not every voxel can be assigned to only a single crack. For example, consider the upper-left most bright green spot in the image from the contest statement. This location is at the intersection of the dark green and blue cracks. At the intersection, either label would be correct, so the "correct" label is ambiguous (ground truth label 2).
Please note that all voxels with ground truth label of 2 are not scored, so your labeling of these voxels is not important. Also, for your assigned labeling, 2 does not represent ambiguity. Instead, it can be used the same as any other number for labeling of a crack.