Problem with Omastar's Attacking Moveset Ratings?

In Omastar's Attacking Movesets rankings, Water Gun/Hydro Pump is ranked a "B" and Water Gun/Rock Blast is ranked as an "A." However, in the accompanying "Offensive Moves Explanation," it says "Hydro Pump is slightly better than Rock Blast." This appears to be inconsistent with the actual rankings. If Hydro Pump is slightly better than Rock Blast, then how can Water Gun/Hydro Pump be lower rated than Water Gun/Rock Blast? What am I missing?