Very likely the logic is set so that Excel does not get confused as to which filename you are referring to when creating formulas from a third file. Suppose I had two open files with name "Yves.xlsx" with different values in cells Sheet1!A1; now, I open a new file and attempt to enter formula =[Yves.xlsx]Sheet1!A1, Excel would not know which source file to pick from; hence the program prevents you from having two different files open with the same filename at once. In this case, Excel is helping us from making mistakes!
Hope that helps!
