That's a good question. It's puzzled me a bit, too.
In my searches on Alexander Chubb, I came across a letter in a book entitled Annual Report of the Commissioner of Indian Affairs, for the Year 1892 by United States. Office of Indian Affairs. On page 168-169 of that book there is a letter from Mary McHenry Cox, Directoress of Lincoln Institution to, I believe, the U.S. Commissioner for Indian Affairs and this letter references a Mohawk Indian Chief Alexander Chubb.
In my known family tree, I have 2 Alexander Chubbs (so far). One is the Alexander that fought in the American revolution b. 1760 - d. 1812 http://www.wikitree.com/index.php?title=Chubb-120&public=1 and the second is his son Alexander b. 1790 - d. ? http://www.wikitree.com/wiki/Chubb-119 Since the date of the letter is 1892, the dates for these Alexanders don't fit.
Now to your question directly, Alexander Chubb is the son of Stephen Chubb and Rebecca Nash and the Nash family is well documented. Stephen Chubb, on the other hand, I'm conjecturing is the orphan warded to the Palmers mentioned in the sources below who subsequently gets land from the Palmer family.
A Digest of the Early Connecticut Probate Records: Hartford district, 1700-1792 edited by Charles William Manwaring
P486 Chubb, Stephen. Court Record, page 56-6 October 1724: this court do appoint John Palmer, Jr., of Windsor, guardian unto Stephen Chubb of Hebron, a minor, 4 years of age. Recog., £ 40.
Barkhamsted, Conn., and its centennial, 1879. by William Wallace Lee, Henry Roger Jones
P25. Old Land Deeds ... In a deed of land by Samuel Palmer to Stephen Chubb, dated February 2d, 1748, Chubb is said to be of this town; also in a quit claim deed from Jacob Gibbs to Chubb, January 13, 1749, Chubb is said to be of Barkhamsted. I have been, I think, credibly informed that Chubb settled near the Green in the Southwest District..
So what was Stephen Chubb's heritage? I found a reference that says Hebron, CT records were destroyed by fire so it's a brickwall for me, so far. He is said to be "of Barkhamsted" which was famous for the Barkhamsted lighthouse, the home of what was probably a scandelous marriage at that time between a Narragansett Indian who went by the unlikely name of James Chaugham and his white woman, Molly Barber. So was Barkhamsted a place on the "wrong side of the tracks"? If so, why was Stephen living there? Here's a link to the story of James and Molly - http://www.ctmuseumquest.com/?page_id=1160
So far I haven't learned anything of Mary Ketcham's past. There are some Ketcham's or Ketchem's documented in Northfield on the Genessee which is where the Chubbs settled, but I have not yet found Mary.
There is also a Chubb family from Vermont. It shares many first names with my Chubbs, but I have not found the connection. Maybe Chief Alexander is connected there.