Sometimes you will see that the result in your IOS simulator or device is a little different from Sketch file. In this Medium article: Why Your App Looks Better in Sketch, you can understand some basic differences, even if you use the same font and size or default settings.
For further investigation, the best is Reveal. Which can help you to look the hierarchy of views and layers, positions, layer settings and you can change these parameters through Layout inspector, which became live on the device or simulator immediately. So you can test your changes before editing it in Xcode, and you save a lot of time with avoiding running the App again and again.
Reveal App also visualize constraints so you can control Constraints during running as well. You can try both App free, but after that I'm sure, you'll become addicted. :)