I think you are saying that you are sewing strips together and they end up looking like a "smile" when you iron them?
they say that has to do with the angle from pushing the iron. we rarely iron directly outward or laterally, but more in a diagonal motion. I guess this contributes to the curve. when I want to be super precise about my strips, i will take my longest ruler and use the straight edge to guide to give it a good finger press. then, I will carefully iron it in place. sometimes you can massage it back in place it its a little bit off, but it's best to prevent it.