Work-Kinetic Energy Theorem Applicability

Hello there.

I've yet to take a theoretical mechanics course (next Spring) but I've been reviewing my intro level class and I've run into some confusion regarding the work-kinetic energy theorem.

My textbooks state that
[itex]W_{net}=ΔKE[/itex]

And this makes sense to me: the net work being zero implies zero net force which implies zero acceleration. However, the books then go on to state that

[itex]W_{ext}=ΔE_{mech}=ΔKE+ΔPE_{g}[/itex]

and I interpret this to mean that the net work done by external forces on a body is equal to the total change in mechanical energy of that body, including both kinetic energy and potential energy (in this case, gravity).

What I can't wrap my head around is why the kinetic-energy theorem gives the net work as the change in kinetic energy but the second equation gives the net work as the change in the entire mechanical energy. I believe that the first theorem applies in all cases, but when is the second applicable?

Thanks for any clarification!