These actually were two different titles. The king of the Germans was elected from the twelve Prince-electors (Kurfürsten). After Otto the Great, it became tradition that the current King of the Germans became crowned by the bishop of Rome aka. the pope, as Roman Emperor. However, at least in the beginning this was no automatism. Thus, not all German kings became Roman Emperor.
These actually were two different titles. The king of the Germans was elected from the twelve Prince-electors (Kurfürsten). After Otto the Great, it became tradition that the current King of the Germans became crowned by the bishop of Rome aka. the pope, as Roman Emperor. However, at least in the beginning this was no automatism. Thus, not all German kings became Roman Emperor.