Designing an efficient and secure authentication scheme is an significant means to ensure the security of IoT systems. Hundreds of authentication schemes tailored for IoT environments have been proposed in recent years, and regrettably, many of them were soon found to have succumbed to security vulnerabilities. In an effort to investigate the underlying reason for this, Wang et al. (at TIFS’23) recently analyzed the vulnerability of authentication schemes from the perspective of provable security. However, we observe that some authentication schemes with sound security proofs and heuristic security analysis are also not resistant to certain attacks, and even those that have been improved several times are still not immune. To explore the deep-seated reasons for security vulnerabilities in IoT authentication schemes, we divide security attacks into explicit and implicit attacks and find that many authentication schemes exhibit security under explicit attacks but are rendered vulnerable under implicit attacks. Further, we propose the relationship between the design goals of security attributes of authentication schemes and implicit attacks, analyze the vulnerability of three typical authentication schemes under implicit attacks, and find that only the security attributes capable of resisting the strongest implicit attacks are secure. Finally, we offer some specific suggestions on how to achieve the security attribute goals.