Albert Barnes Commentary Judges 8:31

"And his concubine that was in Shechem, she also bare him a son, and he called his name Abimelech." — Judges 8:31 (ASV)

Abimelech’s mother was not counted among the wives, probably being one of the Canaanite population in Shechem (Judges 9:28); neither was Abimelech himself counted among the 70 other sons of Jerubbaal (Judges 9:24. Compare Judges 11:1-2).