Ada de Morville - who is Wiliam's mother?

William ll Bertram
Birth: 1157 Mitford Castle, Morpeth, Northumberland, England, United Kingdom
Death: 1205 (48) Mitford Castle, Morpeth, Northumberland, England, United Kingdom

Son of Sir Roger Bertram, Baron of Mitford and Ada de Morville

William Bertram, Lord of Mitford

But when I get to Ada de Morville William is not listed a child. Since Helwise, Ada's mother was born abt 1120, Ada could not be burn before abut 1140, and that makes Ida the mother of William at age 17. Not impossible, but why is William not listed as a child but Ada is listed as his mother?