Are you sure of the Oldham birth? On the 1841 census, Samuel's age is given as 50. Ages were rounded down to the nearest 5 years in this census - so Samuel could have been 50-54 years old - giving birth years 1885/6to 1890/91
Looking at the Family search site there are a number of Samuel Fittons born 1785-1792 - Are you sure the Oldham Samuel is your man? The parents Abram & Sally are given for the 1792 birth. Also using family search there are a number of Abraham/Abram Fittons born in Lancashire. There is an Abraham baptised in 1760 at Oldham - parents John and Ann. Do you have any marriage records for these people that specify the grooms Father?
Just a few ideas - not so easy before civil registration & the 1841 census only gives England as place of birth. Given that Catherine was Scottish, the Scottish naming standards would name the first son after Samuel's father & the second son after Catherine's father. From their marriage, Catherine's father is James (also their second son) - so this could suggest Samuel's father might be William.