Agreed; my initial interpretation was thinking of the tapered edges as comet tails or motion blur, which is the opposite direction to what they want to depict.
I also thought the thick end meant it was receiving flow as opposed to originating it. It's also hard for me to look at the graphs and not see motion, particularly the curved ones - years of reading comics. But reading graphs is an expert activity, and I'm open to the idea that with practice this method could become better.