To describe the continuance of an existing event or state, an initial cause and the persistence of its effect may be relevant. The continuance of non-existence, by contrast, can often be understood simply through the absence of a cause that would bring the event into being.
This distinction helps explain how a negative past can remain circumstantially relevant over a broad interval. It does not create a rigid mechanical rule for every particle of negation. The actual discourse context must establish whether the speaker denies occurrence in a bounded period or evokes an ongoing absence.