C#: Why calling implemented interface method is faster for class variable than for interface variable?

I found this strange behaviour in .NET and even after looking into CLR via C# again I am still confused. Let's assume we have an interface with one method and a class that imlements it:

interface IFoo
{
    void Do();
}

class TheFoo : IFoo
{
    public void Do()
    {
        //do nothing
    }
}

Then we want just to instantiate this class and call this Do() method a lot of times in two ways: using concrete class variable and using an interface variable:

TheFoo foo1 = new TheFoo();

Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
for (long i = 0; i < 1000000000; i++)
    foo1.Do();
stopwatch.Stop();
Console.Out.WriteLine("Elapsed time: " + stopwatch.ElapsedMilliseconds);

IFoo foo2 = foo1;

stopwatch = new Stopwatch();
stopwatch.Start();
for (long i = 0; i < 1000000000; i++)
    foo2.Do();
stopwatch.Stop();
Console.Out.WriteLine("Elapsed time: " + stopwatch.ElapsedMilliseconds);

Surprisingly (at least to me) the elapsed times are about 10% different:

Elapsed time: 6005
Elapsed time: 6667

The difference is not that much, so I would not worry a lot about this in most cases. However I just can't figure out why this happens even after looking in IL code, so I would appreciate if somebody point me to something obvious that I am missing.