A 3-day-old newborn boy delivered by vacuum extraction presents with a large, fluctuant swelling over the scalp that crosses the midline and extends from the forehead to the occiput. The swelling is soft, compressible, and not limited by suture lines. The infant is pale, tachycardic (HR 160/min), and has poor feeding. The structure marked **D** in the diagram is the site of this collection. Which of the following best explains why this condition is clinically more serious than a cephalohematoma?