Because there can be outliers in terms of slowness; perhaps the CPU gets a hickup, the GC runs an abnormal amount of extra times, the kernel decides to reschedule stuff, whatever, I don't know much about these things.
There can't be outliers in terms of fastness; the CPU doesn't accidentally run the program much faster.
It's the same logic both ways. The OS runs 10 background tasks on average at any given time. On one of your runs it was doing 15 things, while on another it was doing 5 things. They are both outliers.
