The thing with adventurer ranks is that they don't track/directly translate to actual power, but rather are a metric of tracking achievements and progress in the guild.
Granted, you would need a certain amount of power to complete the higher ranking quests and continually doing quests that are appropriate for you rank would result in higher power levels, but you could just do what MC is doing and simply grind monsters and you will get stronger over time although that will not affect your rank and thus result in your Power level not matching your rank.