Mary E. Rigney

My grandmother was born to Elijah Rigney and Margaret Elizabeth Shupe in 1872. I found Elijah and Margaret recorded in the 1880 Montgomery County, Virginia census. This would be the first census that Mary might appear. That said, there is no entry for a Mary E. Three children listed are Malinda age 12, William age age 5 and Admonia M. age 7. It is highly probable that Admonia is my grandmother's real name. The 1880 census is the last census in which I find Elijah. I found Margaret Elizabeth in the 1900 census in Tazewell County, Virginia but no children attached to her.
Again a mystery with my grandmother. Why did she move to Newcomb, Tennessee or the area? When did she move and how did she get there?