This could be a bug, looks a bit like the window border counts towards the specified size, when in fact only the client area should. If you are changing window settings at runtime, this might also be a factor. Feel free to
file an issue on this and we'll look into it - make sure to include source code, if the repro case requires any!
If you're starting a new project, or are still in the very beginning, you could also
look into the v3.0, which offers a different way to lock your game at a fixed resolution, while at the same time allowing free window resizing. In the application settings, you can now set a virtual resolution, which will be emulated regardless of window or screen size, both in the editor and the runtime.