Born before their marriage. Could some other woman have been his mother?
1880 U.S. Census (Ancestry.com): Persy Foster (33 b. England) living in Elmira, Salano Co., CA with wife, Amy (33 b. VA) and 4 kids: Maggie (13 b. Kansas); Charles (12 b. Kansas); Lillie (3 b. CA); and Gracie (2 b. CA).