Plays are ranked by difficulty based on where the ball is hit, what type of batted ball it is (fly ball versus line drive, for instance) and whether the ball was hit soft, medium, or hard. The difficulty factor is determined by how often that type of play is made.
If a player makes the play look easier or harder than it was, that has no bearing on the difficulty factor. JBJ making a play look routine, and another CF having to dive to make the same play would get the exact same credit.
With Statcast data now available, defensive metrics will become more and more accurate.