If the Hero Association thinks that King is the strongest man alive, why isn't he ranked higher? King is many times referred to as the strongest man alive and the Hero Association stats give him a ranking of 10 in everything: Stamina, Intelligence, Justice,     Endurance, Power, Popularity, Effectiveness and Fighting Ability. So, if they think he's perfect and the strongest hero, why didn't they rank him higher than #6?
Adding onto @Gary Andrews30, the reason that King wasn't ranked any higher was because of the simply outstanding heroes ahead of him. 

.6. Bofoi (aka Metal Knight), he helped create the newer Hero Association and he literally has the skills to manage extremely advanced technology. And as Gary said above, rank isn't determined by strength, they look at other things. King has almost never responded to his hero calls (calls for help by the hero association) and only comes when there are the biggest of crises (ie. Monster Association) and that was only due to Saitama coming along. However, since Bofoi doesn't respond much either, the deciding factor would probably be the contribution to the Hero Association. As mentioned before, Bofoi helped construct the new Hero Association building, while King hasn't done much in that retrospect(This isn't backed by much evidence because Bofoi doesn't have much character time and indications).

.5. Child Emporer, his overall skills include extreme intelligence and technological mastery. He is most likely not as skilled as Metal Knight, but a possible factor could be his willingness to help out the Hero Association and his strategic leadership as shown when he led the S rank heroes into the Monster Association. Therefore, because of his active participation vs King's declination of emergency calls, Child Emporer is ranked higher than King.

.4. Kamikaze (Atomic Samurai), need I explain this one? Although mentioned in the Webcomics that 


  Kamikaze believed that King cut the apple (a test of strength) so fast that the molecules rejoined


However, as readers, we obviously know that King if actually faking and doesn't know what he is doing. However, Kamikaze has connections and pupils, which makes him viable and due to his immense prowess in katanas he is considered extremely dangerous and powerful. His pupils that are A rank 2, 3, and 4 (remember, A class high rankings are already considered powerful) contribute with Atomic Samurai and if Atomic Samurai, the proud Samurai he is, happens to dislike something or is displeased with his rank, the Hero Association would most likely lose his pupils along with a great warrior. Not only that but Atomic Samurai also has connections with other sword dojos as shown when they meet up and discover that one of them had been monsterfied with the monster hearts. One of the Masters mentioned "I will mobilize my pupils", so notice the plural of the word "pupils". This means that because Atomic Samurai already carries influence among the top-ranked A heroes and also has other sword dojos backing him up, the Hero Association could not afford to lose someone like him. Compared to King, Atomic Samurai holds to much influence and power/skill to be ranked lower.

.3. Bang (Silver Fang)
Simple, Bang is wise. He recognized Saitama's power right away and even mentions that Saitama is more powerful then him. Bang also has mastered one of the most deadly martial arts techniques, Fist of the Flowing Water Crushing Rock. (Just take Garou as an example). Moreover, Bang always showed up, no matter what happened. For example, at the start of the series when a meteor was falling, only Bang, Genos, and later on Metal Knight and Saitama showed up. Bang calmly, calmly looked at the meteor that could literally destroy him and just chilled. Genos, the Demon Cyborg only came because he was worried about master Saitama being bothered by the meteor. ( I assume that he would've just yeeted outta there if Saitama wasn't there), and Bofoi (Metal Knight) only went to test new technology. So not only does Band cough responds to the calls, no matter how dire they are, but he also uses an extremely powerful martial arts technique that not many people can withstand.

.2. Tatsumaki (Terrible Tornado) Tatsmuaki doesn't really need to be explained. Yes I do realize that she doesn't respond to calls much either, but she is just simply too strong. My theory is that if she can float, how would the "Strongest Man on Earth" even touch her? We have never seen King's feet leave the ground (Just gonna assume that King can't fly or levitate). So based on that, Tatsumaki simply overpowers the "Strongest Man on Earth", not based on physical prowess, but by psychic raw strength.

.1.Blast
Not much is known about Blast even int he Webcomics, but there are many theories that I won't explain because of lack of evidence. However, he has been described as, 
"Fubuki mentions that Blast is "the one at the top of all heroes" during her talk with Saitama in his apartment, putting Blast alongside King in terms of power."

Buuuut we all know how powerful King really is right....?
Wait, you're telling me that after spending ~1 hour writing this that King actually is the strongest? WDYM? ~~~~~    ~~~~~~
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:3