法華義疏 8.329

8.329

此明根性不定。一者、可退為凡夫；二者、可住為二乘；三者、可進為菩薩，以不定故所以標欲也。

This clarifies that their disposition is not fixed. First, they could retrogress to become ordinary persons; second, they could abide as the two vehicles; third, they could advance to become bodhisattvas; because it is not fixed, it therefore marks it as 'desiring'.