कल्पान्तरे विश्वासटिप्पन्यः

A kShatriya had maximal theoretical legitimacy for kingship. This led rulers to claim kShatriya status, and some brAhmaNa-s (like gAgAbhaTTa - see ministership page) to strive to make space for them within the dharmashAstra framework.
In the absence of de-jure kShatriya-s, ability and heridity still brought maximal available legitimacy.
Principles of proper governance very much applied to kings irrespective of varNa status.