I guess it depends on how you're adding those 50% together: add three bullets to a 6 chamber revolver for a game of Russian roulette and 3 bullets = 50% chance of death. If you take two tries, sure, that's not 100% chance of death. But 3 bullets = 50% chance, so if you add another another 3 bullets then those percentages combine quite well for 100% chance of death :)

(assuming your aim is good & lethality on hit is 100%)

Nah, it is a issue with units for me. Speaking primitively: % and ‰ look weirdly and add weirdly - they are easy to remember.

But when some unit looks like a meter or gram (especially using the micro- prefix here), it should behave accordingly.

Of course, even your example units don't always behave like you want. For velocities, 150 megameters/second + 150 megameters/second ≈ 240 megameters/second.

Sorry, can you explain why the velocities should add that way? I've always just seen velocities add like regular old vectors, so this looks very strange.

Relativity. If they could add that way, you would get 300 megameters/s, which is breaking the speed of light.

That'd be special relativity for you - nothing ever exceeds the speed of light, hence a corrective factor needs to be applied.

https://en.wikipedia.org/wiki/Special_relativity

Relativistic effects.