Isn't the ribbon part of the widget-library included in the Windows APIs? If you use widgets from the same library it will obviously look the same.
Although this case is a bit extreme, the order of the buttons and everything is a carbon copy, if the title bar was removed i would have a hard time telling which one is Corel and which one is Excel, and I'm a daily Excel user.
MS Office is used as an example of the "right" way of doing UX on Windows. It includes button order, names, etc. Under these constraints all decently made spreadsheet programs should basically look the same. "Carbon-copy same" is improbable in practice, but "mostly same" is what's expected.
Although this case is a bit extreme, the order of the buttons and everything is a carbon copy, if the title bar was removed i would have a hard time telling which one is Corel and which one is Excel, and I'm a daily Excel user.