SAP states that "The number of posting periods must be the same for company code and controlling area. A variance is only possible in special periods. The period limits in the fiscal year variants must correspond to each other, which means the fiscal years may not overlap."
This translates to the point that the companies assigned to a Controlling Area can have different fiscal year variants assigned to them provided they only have different special periods. But when you try to assign companies with different fiscal year variant to a Controlling area, you get a error.
Although, once you have assigned the companies to the Controlling area and go back and change the fiscal year in one of the assigned company codes, it doesn't give you an error, which fully corresponds to the explanation given by SAP.
Why are there differing controls? Can someone please explain? Attached screen shots for your reference.