So think of it kinda like a snakey bow, the limbs can snake back and forth all they want as long as the tips line up and your string is centered on the handle. So draw an imaginary line from the point to the nock, lets use the arrow in a vertical position for this image, now even if the nock is straight up and down if it is off to the side it will not spin true and you'll have issues, or even if it is centered but at a slight angle off to the left or right you'll have issues, but if the tip and the nock fall right on that imaginary line the middle bit can be wonky/crooked, and you're right about the mass being more or less balanced because you can't get away with TOO much or it won't be balanced/spin true, but within reason it doesn't have to be a perfect carbon straight arrow to be a great arrow.