If it’s only off a little bit, I wouldn’t stress. Sometimes it’s just the way things are cut out, that they don’t end up perfect. When pinning, match up the ends together and pin them first, then ease everything in between and it might just work itself out. If you need to see an example of easing and pinning you can watch this tutorial: https://www.professorpincushion.com/professorpincushion/fabric-easing/