Kikan IKEDA surmises that this was because the compiler considered the 'Agemaki' (Trefoil Knots) chapter a 'parallel chapter' to the 'Shigamoto' (Beneath the Oak) chapter and therefore did not give it a number.
池田亀鑑はこれを総角(源氏物語)を椎本の並びの巻にしてしまい巻序を示す数字を書かなかった。