I don't know if this helps but on the 1881 census there are 2 other boys named Garrett on the training ship "Exmouth." Both are listed as paupers. Charles Garrett born 1867 Hackney, Middlesex William Garrett born 1868 Poplar, Middlesex.
It has been my experience that the place of birth is not always accurate for children who where placed in the care of local councils (workhouse) or other facilities.
It might be worthwhile to get the records of these other Garrett children.