In addition to the personal preferences, one of the other reasons is the actual lines; In the case of the Douglass' in my tree, the surname as I understand it was originally "Douglasson" - ie: the son of douglas. There is also the royal "House of Douglas" from Ireland. The second "s" was kept to help distinguish the two lines from each other.
My Douglass' in some cases lost the second "s" with census' in the late 1800's early 1900's. That depended on the census taker and the family head.
My GGrandmother used both spellings. Her marriage certificate has "s". One or two of the census' has "ss".
When or if I get a birth or baptism record, I'll very likely go by that as for the family name.
For the research I've done regarding my family, we're of the "Douglass'"... Just a bunch of farmers, carpenters, laborers and shop keepers. :)
Hope that didn't add to the confusion.