Why would System.Type.GetType("Xyz") return null if typeof(Xyz) exists?

I have come across a strange behaviour in my (huge) .NET 4 project. At some point in the code, I am referring to a fully qualified type, say:

System.Type type = typeof (Foo.Bar.Xyz);

later on, I do this:

System.Type type = System.Type.GetType ("Foo.Bar.Xyz");

and I get back null. I cannot make sense of why this is happening, because my type name is correct, and I have checked with other types and they get resolved properly. Moreover, the following LINQ query finds the type:

var types = from assembly in System.AppDomain.CurrentDomain.GetAssemblies ()
            from assemblyType in assembly.GetTypes ()
            where assemblyType.FullName == typeName
            select assemblyType;

System.Type type = types.FirstOrDefault ();

Are there any reasons why System.Type.GetType could fail?

I have finally had to resort to this piece of code instead of GetType:

System.Type MyGetType(string typeName)
{
    System.Type type = System.Type.GetType (typeName);

    if (type == null)
    {
        var types = from assembly in System.AppDomain.CurrentDomain.GetAssemblies ()
                    from assemblyType in assembly.GetTypes ()
                    where assemblyType.FullName == typeName
                    select assemblyType;

        type = types.FirstOrDefault ();
    }

    return type;
}