First of all, we are not travelling in a straight line, we must travel forward, turn 90 degrees right, go forward, then travel 90 degrees right again.
Second, the distance may or may not be greater, that's not the point. When you don't do an arc, you have to stop, turn, then move, and then you have to accelerate back up to your top speed. When you do an arc, you apply power to the motors continously, and thus you aren't jerky and you save some speed.
Now, it may be different depending on drive train issues, and robots, but it's not as simple as "the shortest distance between two points is a straight line" (unless you are 179 and you can just climb over the rail, thus going in a straight line!
)
Stephen