Albert Barnes Commentary 1 Chronicles 25:8

"And they cast lots for their offices, all alike, as well the small as the great, the teacher as the scholar." — 1 Chronicles 25:8 (ASV)

As well the small as the great - Compare 1 Chronicles 24:31. The lot was not applied indiscriminately to all 24 courses, but was only used to determine which course of Asaph, which of Jeduthun, and which of Heman should be taken on each occasion. Asaph was given precedence over his brothers, and his four courses were assigned the first place and then each alternate place. Jeduthun took rank next and received alternate places, first with Asaph and then with Heman, until his courses were exhausted. After this, all the later places necessarily fell to Heman, whose courses continue without interruption from the 15th.