Light travels at, more or less, 186,000 miles per second. Thus, 186,000 miles is one light-second.

It's just one of those numbers.

Like if you'd said ``I just got my car from the lot and it'd only been driven 3.14 miles'' or ``My spaceship's overdue for an overhaul. It's been 6.022 x 10^23 miles'' or ``They say that the average family has 2.71828 children''.