Meckel's Diverticulum - Antimesenteric Ileum

A 3-year-old boy presents with a 2-day history of painless passage of maroon-colored (currant-jelly) stools mixed with fresh blood. Vital signs show tachycardia (HR 140/min), pallor, and mild hypotension. Upper GI endoscopy and colonoscopy are normal. A Technetium-99m pertechnetate scan demonstrates focal uptake in the right lower quadrant appearing simultaneously with gastric uptake. At laparoscopic surgery, the structure marked **A** in the diagram is identified on the antimesenteric border of the ileum, 55 cm proximal to the ileocecal valve. Which of the following best explains why this patient presented with painless lower gastrointestinal bleeding?