The NS UI looked really odd to me until relatively recently having grown up on Mac OS Classic, Win9x. Now I look at that and think, damn.. that aged well.
It's probably lost to history now, but portions of Windows 95 drew partial inspiration from earlier versions of NEXTSTEP, such as the window buttons and titlebar.
So OpenStep for Windows NT ran under windows NT and it looked like Win32 GDI but it wasn't Win32 GDI. It was still display postscript but the GUI elements (NSButtons, etc) were themed to look like Windows NT instead of NextStep. For whatever reason they were included in Mac OS AppKit as well.