
The measurements given are exact and 3 mm bleed should be added on each edge to allow for trim.
When the artwork is trimmed, slight variations can occur so the 3 mm is there to to avoid a white line being shown on trimmed edges.
No text or detail should be placed closer than 3 mm to trimmed edges or folds, this ensures no loss of detail etc in the folding, trimming process.
The spline is an exception as you only have 3mm space.