If you open the typeLib C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscorlib.tlb with OleView for example, and navigate to the _Object interface (not the first _Object one, the dispinterface, but the second one) you'll see this:

So, the .NET method Object.ToString() is declared to COM/Automation clients as

[id(00000000), propget, custom(54FC8F55-38DE-4703-9C4E-250351302B1C, 1)]
HRESULT ToString([out, retval] BSTR* pRetVal);

Which means it's seen to COM clients that understand this "syntactic sugar" (VB/VBA/VBScript/JScript/.NET, etc.) as a Property (propget + out + retval) named ToString that returns a String (BSTR).

Now, id(0) means it's the default property, because 0 represents DISPID_VALUE, a special well-known id.

And lastly, VB/VBA VarType's documentation states this:

If an object has a default property, VarType(object) returns the type of the object's default property.

(which I always found a pretty strange design decision...)