Well, think abt it as this way. When you create a new class, without deriving from any other class, it is automatically made a sub-class of System.Object. However, when you subclass from another class (say Base), you are inherently sub-classing from Object because of multi-level (NOT multiple) inheritance (because, Base is derived from Object).
Thats my take on it. Would be happy to know more and to know if I am missing something.