From gedcom exported from FTM2005, I have individuals with multiple dates of birth from different sources. The Index displays the earliest date, whether or not it is the preferred date.
Example, on this test file, the preferred field is b. 1900 Cabrach, which is indeed given precedence in the list of facts.
Joe BLOGGS
M, #I01, b. abt. 1898, d. bef. 1960
•Father: ?
•Mother: ?
•Birth: 1900 Cabrach
•Birth: abt. 1898 Scotland
•Death: bef. 1960 Birmingham
•Occupation: farmer 1945