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Why does a tornado vortex signature not always mean that a tornado will form?

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Doppler radar
Weather forecasting Doppler radars, such as this one operated by the U.S. National Weather Service at New Underwood, South Dakota, extract the Doppler frequency shift of the echo produced by a moving target by noting how much the frequency of the received signal differs from the frequency of the signal that was transmitted.

A tornado vortex signature (TVS) detected on Doppler radar indicates a region of intense, concentrated rotation within a thunderstorm. However, this signature does not always evolve into a tornado because the presence of concentrated shear is only one component of the complex atmospheric process required for tornado formation.1

While a TVS identifies a rotating updraft or a potential tornado coreā€”a region of low atmospheric pressure bounded by maximum tangential windsā€”the transition from a rotating column of air to a surface-reaching tornado depends on several evolving factors within the storm's structure. For a tornado to form, the diffuse spin must be successfully concentrated and maintained as the storm progresses through its distinct developmental stages.1

Because the radar indication of intense rotation is a measurement of wind shear rather than a confirmed physical funnel, it is possible for the rotation to dissipate or fail to intensify sufficiently to reach the surface, which is why the TVS is not a guaranteed precursor to a tornado.1