C# NUnit unit test not using correct method return to compare

I have the following class

public static class MyClass
{
    public static double Converter(int mpg)
    {
       return Math.Round((double)mpg / ((double)36 / 12.74), 2);
    }
}

And NUnit unit test

[TestFixture]
public class ConverterTests
{
    [Test]
    public void Basic_Tests()
        Assert.AreEqual(8.50, MyClass.Converter(24));
    }
}

My Unit tests fails with

Expected: 8.5d
But was:  8.4900000000000002d

When I debug the method return 8.5 so where does the unit test get the strange number from?