I would guess that when you don't precut to specific lengths and make them fit, you will get the same results as when you don't measure and precut for borders. The feed dogs on the sewing machine can pull the bottom fabric thru at a different rate than the top, resulting in wonky blocks. I know not everyone has this problem, but it might be part of it. I know I made much better log cabin blocks when I precut to exact length. The next log cabin blocks I make will be the paper piecing method.