why the track of the bullet is showing at all,

That's not the track of the bullet. That is THE bullet. The smeared line is how far it traveled while the "shutter" was open. The line starts when the shutter first opened and ends when the shutter closed. That is how fast moving objects look in photographs. With kids running or cars driving by, it often look more like its out of focus because the actual movement is fairly small. But it's the same effect.

--Peter

PS - I say "shutter" because many digital cameras do not have an actual shutter any more. They record the light hitting their sensor for some period of time, rather than having a physical shutter open and close to expose film for a period of time. But the function is the same: to record how much light hits the sensor/film over some (typically very small) period of time.