This is just an installer quirk. if you were to re-run it, you’d notice that when you pick a folder it adds ‘gratuitous space battles’ on the end just after you’ve browsed. its there right in front of you before you click on ‘ok’ or ‘install’, but you probably assume (as everyone, including me would do), that the path you browsed to is the path in the text box, but it just aint so.
You can just select the path at that point and remove that extra bit.
Its annoying, but the installer does it because thats the default path for most copies of the game, and annoyingly I haven’t got any way to detect if its a steam copy and change it automatically.