The whole context:
So I decided to make some meat pies. In a short time I was busy mixing butter and flour and my hands were soon covered with sticky pastry. At exactly that moment, the telephone rang. Nothing could have been more annoying.
According to the concept of the third conditional tense, "Nothing could have been more annoying" should be an unreal situation, but what is the real situation in this case?? I can't figure out the logic in it.