Single Dark Endo Treated Tooth

Can someone explain to me why would you need to use high concentration of hydrogen peroxide to bleach ENDO treated teeth? This is a before and after (1 week) single dark tooth external bleaching using 10% carbamide peroxide #opalesence #ultradentproducts Patient is now ready for full maxillary arch (10 teeth) night-guard bleaching. #thinkC #thinkconservative #rds #restorativedentalseminars #marioromerodds