Either the family had JUST moved to Brandon in Aug 1870 when the census was taken, or Mattie was born on a visit back to Vermont. Or, they moved close to the time of her birth, and 10 years later when asked they said Vermont when in fact she had been born in Brandon.
740
From the Harrietstown Coreys cemetery (transcribed at http://freepages.genealogy.rootsweb.com/~frgen/harrietstown/coreys.htm):
DUKETT
John DUKETT   1834 - 1922
Nancy GRAHAM   His Wife   1841 - 1923
Mattie Jane   Daughter of John & Nancy DUKETT   died Aug 23, 1880   age 9 yrs. 4 mos. & 2 D
  "A precious one from us has gone
  A voice we loved is stilled
  A place is vacant in our home
  Which never can be filled"   - by her sister