I found early in my quilting career that accurate cutting is the key to getting seams to match. In the old days when I used templates, my seams rarely matched up. Now that I rotary cut just about everything, I don't seem to have that problem.
Another secret I will pass along is that not all fabrics sew the same. Some will creep a little bit. Have you noticed when you get to the end of the seam one fabric suddenly extends further than the other layer? That's called creeping. The next time you sew that fabric, put that fabric on the bottom when you sew the two strips together. The feed dog will prevent the bottom layer from creeping. Also, some fabrics have more stretch than others. That will affect the piecing too.
And keep in mind that you are a human, not a machine and it's ok that your seams don't match perfectly. If they're way off, something's wrong and needs to be fixed. But to be off a little bit, it's ok. Many times quilting can correct or hide our many mistakes.
If anyone says anything about it, just simply smile and say "thank you for noticing, I planned it that way".