Hey, Runatic!
A half marathon is 21.0975 km: exactly half of 42.195 km. At the “21” marker, you still have 97.5 meters to go. Distance regulations
At 5:00 per kilometer, that's almost half a minute. A rather noticeable half-minute if you're chasing a sub-two-hour finish.
“I ran twenty-one” and “I ran a half marathon” differ by a small but stubborn tail. Even half a marathon refuses to round itself down for our convenience.