Speaking as someone who spends a lot of time poking around the RimWorld codebase, it’s mostly because the game’s UI tends to round things to pretty then up for the player. Under the hood, infections are usually some ugly decimal number (like .9956424) that visually round up, it isn’t until that number is fully at/ about 1 that the death by infection mechanism is triggered.
Speaking as someone who spends a lot of time poking around the RimWorld codebase, it’s mostly because the game’s UI tends to round things to pretty then up for the player. Under the hood, infections are usually some ugly decimal number (like .9956424) that visually round up, it isn’t until that number is fully at/ about 1 that the death by infection mechanism is triggered.