Because the term refers not to any exact shape, but rather to the abstract concept of a hull shape optimized for underwater travel, and more specifically the physical manifestation of this idea in actual submarines as limited by material sciences and the design requirements placed upon the vessel, whether or not any specific vessel exhibits a teardrop hull depends on the subjective interpretation of the term itself.
The concept of a teardrop hull is not about a specific geometric shape, but rather an abstract idea of a hull design optimized for underwater travel. The physical embodiment of this concept in submarines is influenced by material limitations and design constraints.