One way to look at the difference is that a "natural Ebola" almost certainly would have spread at some point anyways, and so it wouldn't affect the total number

One way to look at the difference is that a "natural Ebola" almost certainly would have spread at some point anyways, and so it wouldn't affect the total number of world deaths in the end, just make them happen some number of years sooner. Whereas an "artificial Ebola" would never have existed without it being intentionally created, so all the deaths aren't just time-shifted, they wouldn't have happened otherwise. They're new.