Idiosyncratic risk (or individual risk)
\ɪdioʊsɪnˈkrætɪk\ \rɪsk\
The risk that certain members of a population live significantly longer (or shorter) lives than that predicted, driven by the natural variation in a population (where some people may just be (un)lucky with how long they live).
This risk can still be prominent even if the underlying assumption for longevity is ‘correct’. From the law of large numbers, idiosyncratic risk will decrease as the size of the population increases and as the concentration of risk decreases.