Think of potential this way- High potential means that category would move up faster than his other categories, but it's still going to increase at a rate based on WE and PT.
Example, a guy could have 1 WE, 1 BH, and High BH potential and maybe get up to 5 BH by the end of the season if you're lucky, whereas a guy with 75 WE, 50 BH and medium potential could increase 10 or more.