Well, we don't know for sure, and we may never know. However, there are some things we do know that contribute to coming up with an educated guess.
Schools were few and far between in the late 1700's and early 1800's and many children didn't attend school on a regular basis. Talcott probably could not read or write. He left Fairfield and started migrating west. Perhaps when he was married, or when his first child was born, or perhaps his first experience with a census taker, he had to provide his name, but could only say his name. He couldn't write it. The person writing the name for Talcott spelled it the way he/she heard it -- phonetically. This was customary in those days, even as late as the early 1900's.
The Irish might have pronounced "Davern" as "da-van," "duh-van," or "duh-varn" with two distinct syllables and a silent "r", resulting in spelling the name "Devan". The capital "V" came later; it gave the appearance of two distinct syllables in the name. The capital "V" is prominent in my great-great grandfather Erastus N. DeVan's writings. (See Note.) But not all DeVans spelled it "DeVan"; Ralph P. signed his Civil War letter "Devan" with a small "v".
In Ireland, the names Devane and Devan are both pronounced "duh-van", with two syllables. In American English the names are typically pronounced "dee-vane" and "devin", two very different sounding names.
Note: In his early adult years Erastus N. could not read or write, which reinforces my belief that Talcott also could not read or write.
DeVan is a variation of Devan, which is a variation of Devin, or Devine. Devin is from an Anglicized Irish surname derived from either of the Irish surnames Ó Damháin (which means "descendent of Damhán") or Ó Dubhán (which means "descendent of Dubhán"). The given name Damháin means "fawn"; the given name Dubhán means "little black one". Note that "fawn" is also a dark color (for a person).
Davern is an Anglicized form of the old Gaelic O'Dabhoireann. The Gaelic prefix "O" indicates
"descendant of", plus the personal byname Dabhoireann, which in its original form Dubhdabhoireann denotes "the dark or swarthy one of the Burren", from "dubh", black and "boireann", the Burren, (County Clare). The "Wars of Torlough" mention one, Coradh mic (son of) Dabhoirenn in 1317. The surname first appears on record in the mid 14th Century. The Daverns were described as "a learned Brehon family", seated at Lisdoonvarna where they had a literary and legal school.
Note that DeVan and Davern have "dubh" (black or dark) in the original Irish names. Read more about the Davern surname at this site.
Also read this very interesting Wikipedia article about surnames.