Actually I think definition d may be most relevant. Differential is a mathematical (calculus) term, literally meaning the derivative of some variable (position) with time, which is formally a rate. So "differential deaths" may be implying a difference in death *rate* between two time points. I.e., how much an observed variable changes over the course of a finite time period.

Example: if you make a point observation of two things, that's observing a difference (this bread is more moldy than that bread). If you make an observation of how two things change over time (this loaf of bread turned more moldy between yesterday and today than that loaf of bread), that's observing a differential.