Lethality is the effectiveness of desired outcome. Was the target eliminated?
Collateral is the side effects of the desired outcome. Was anyone or anything else eliminated?
They are two different metrics. Consider two contrasting analogies which break your understanding:
Ballistic missile: high lethality, high collateral probability.
Polonium tea: high lethality, low collateral probability.
About 20 years ago, I was working in the defence sector, on guidance systems, as an engineer. Sat through many a powerpoint about how collateral could be marketed away. Turns out they won that battle eventually...
Incidentally I bailed out when I saw what a fucking mess of a world I was creating.