When you take a screenshot of a non-rectangular window (e.g. iOS Simulator), the background behind the window is also captured. Other screenshot utilities capture these windows with a transparent background. See attached comparison.
Another possibility is to actually capture an area larger than the phone. But before you do, ensure the phone image is in front of something like an open instance of Windows Notepad with nothing appearing on the Notepad area. That would give you a nice clean background with which to work. After you capture the area, just flood fill the white background with transparent. Then use the Auto Trim to pull all edges in to the surrounding phone image.
I submitted similar feedback, but got zero bite. I want to capture bezels and all irregular shapes. Here's my post: https://feedback.techsmith.com/techsmith/topics/capturing-rounded-corners-and-other-irregular-shapes