I think that is actually a bad example and article by Joe. Making any observations on improvement on two or three days of training period is obviously meaningless. Over longer period of time, say month and more, sure, you can see differences in your efficiency factor. On a day to day basis, there are so many factors, from external, like temperature, humidity, to internal - sleep quality over last night or several days, food eaten, workouts done before, etc that impact heart rate.
I would suggest reading another article by Joe, which, in my opinion, is much better representation of his knowledge on this topic.