If someone is really determined in one’s devotional service, why would that person fall?

From Promila Mataji

Lord Krishna says in the Gita, Arjuna great yogis fall too mamamaya duraritya. And therefore, surrender to Me, He says. So, does this shloka mean that it’s not necessary that the one who is determined in devotional service is also a fully surrendered soul? I ask this because why would a fully surrendered soul commit a mistake?