The Paradox of Section 87: Why It's a Puzzle

I am seriously getting my brain tangled up with Section 87 of the Indian Penal Code. How can it be that the court can take into account the voluntary act of an insane person when deciding on liability? Isn't that going against the very grain of the insanity defence? According to this section, if an insane person does a voluntary act, the court can still hold them liable. But isn't a voluntary act by definition an act done with a clear mind? It's making me think that the whole concept of insanity and liability is being completely turned on its head. Can someone please enlighten me on this? Why does this section exist and how does it play out in court?