This is because the block itself has a left-handed orientation. If you select additional negative determinant histogram bars, they will appear to fill an entire block. The left-handed blocks occur from bottom-up operations such as creating blocks, merging vertices and entire topologies, and copying blocks. They are easily made into right handed blocks by using “Blocking [tab]>Block Checks>Fix Inverted Blocks.” These can also be fixed by writing an unstructured mesh and using “Edit Mesh [tab]>Check Mesh>Volume orientations.” When it finds orientation problems, choose the automatic “fix.”