Why does List<T> implement IList<T>, ICollection<T> and IEnumerable<T>?

Yes, it makes no difference in this case. In some cases it can make a difference, if you're using a base class which already implements an interface but you wish to reimplement it yourself explicitly - but in this case there's no base class (other than the implicit object) and it would have behaved exactly the same way.

Contrary to my recollections, I don't believe there's a difference in the way the class is represented in metadata whether the code explicitly declares all the interfaces or not. Here's an example:

interface IFoo {}
interface IBar : IFoo {}

class FooBar1 : IBar {}
class FooBar2 : IBar, IFoo {}

Both ildasm and Reflector show the same information for FooBar1 and FooBar2... it shows both of them implementing IBar and IFoo.

In other words, we can't tell whether the original source code for List<T> actually specifies all the interfaces or not. Maybe it does, maybe it doesn't - but it doesn't matter either way.

EDIT: For completeness, I also checked the cases where you're extending two interfaces with another interface. I can't find a difference in the metadata in that case, either. I'm sure I remember some situation in which it was evident, but I can't find it now.

Yes it would. IList<T> itself implements the other two.

The object browser shows you all the interfaces the class implements, whether directly (IList<T>) or indirectly (ICollection<T>, IEnumerable<T>, through IList<T>).

That's not the way it was actually coded behind the scenes. That's just what tools like Reflector show you when it turns the IL back into C#.