Ah right. Well, I think it's just trying to indicate to us that the number of lives in the investigation will only be known afterwards/in retrospect. This is generally the case in actuarial mortality investigations, eg if we work for an insurance company and are studying the mortality of policyholders aged 60, we don't know how many people we will observe in advance, as we can't know whether someone aged 60 will take out a policy in the future. Doing the analysis in retrospect then means that N is known, rather than random.
(In certain other contexts, we might be able to specify N in advance, eg if we're conducting experiments on the survival of cells in a laboratory, we might set out specifically to run 100 such experiments.)