Albert Barnes Commentary 1 Chronicles 24:31

"These likewise cast lots even as their brethren the sons of Aaron in the presence of David the king, and Zadok, and Ahimelech, and the heads of the fathers` [houses] of the priests and of the Levites; the fathers` [houses] of the chief even as those of his younger brother." — 1 Chronicles 24:31 (ASV)

The principal fathers over against their younger brethren—that is, all the Levitical houses enumerated drew lots for their duties on equal terms, with the older families having no advantage over the younger ones. Just as there were 24 courses of the priests, so we must suppose that there were 24 courses of the Levites, even though the number of families listed in the text is 25 (1 Chronicles 23:7–23; 1 Chronicles 24:20–30).