Necessity says the contrary cannot obtain under the given qualification. Permanence says the predicate in fact holds throughout the stated period.
Suppose a particular lamp remained lit throughout its short life. “It stayed lit its whole lifetime” reports continuity, but being lit did not belong to the lamp's essence; it could have been extinguished. In contrast, “A human is an animal” expresses a necessary genus–species relation.
Thus, even if two claims mention the same span of time, they need not express the same modality. Continuity and necessity demand different analyses.