3 mm Bleed:
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.

The above image is displayed as a printers pair, pages 4 & 1 side by side, page 4 being on the left and
page one on the right. The total dimensions including bleed for both pages would therefore be:
Width: 248mm (Width page 4 + page 1 + 3mm bleed left + 3mm bleed Right)
Hight: 126mm (Hight of page plus 3mm bleed top + 3mm bleed bottom)