https://www.ancestry.com/family-tree/person/tree/117286476/person/182108602572/facts
Note: Mary is listed as Mary Archer Anderson in this Ancestry Tree. Was she perhaps adopted by John Archer who named her in his will? John Archer did not specifically name Mary as his daughter, so it is possible she had other parents. Additional research is necessary. Her son being named John Anderson Ross may provide a clue.