‘Because the thing about the Buffy universe is that the population of vampires is reasonably stable. There are a fair number of vampires around, but not enough to overwhelm the earth. But as it turns out, if you look at that chart above, there’s a very narrow vampire-population window at which equilibrium can be kept.
That’s because powers of two increase slowly at first, then at a hellacious rate. Think of it this way: According to the numbers calculated by the academics, at month five in the year 1600, there are only 16 vampires. That’s such a paltry number than any self-respecting slayer could quickly dispatch them in a few evenings, and the vampire menace would permanently be extinguished. But at month 12 — only a few months later — the number of vampires, unchecked, rises to 2,048. That’s probably too many vampires for a slayer to squelch in a single month.
So the really sweet spot seems to be months eight to ten — when the vampire population would range from 128 to 512, respectively. [..]’