Why are some pure Fighting-type Pokemon weak to Ground-type moves while others aren't?

Question

Notice how some Pokemon who are purely Fighting-type weak to Ground-type moves while others aren't. For example, if a Pokemon used Earthquake on Hitmonlee, who is a pure Fighting-type Pokemon, it's super-effective. However, if it's used on Machamp, who is also a pure Fighting-type Pokemon, it is neutral. May someone explain?

Comments

I was playing Pokemon HeartGold and I didn't make a video yet.

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You’ll have to provide a video before the 7 day mark of this post, or else we’ll close it under rule 1.4.

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I stand corrected. I am wrong about how some pure Fighting-type Pokemon are weak to Ground-type moves while others aren't. I have uploaded video evidence of me being wrong. https://www.dropbox.com/home?preview=Video.mp4

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It says that the file doesn't exist.
That's not how that works, you need to create a shareable link for the file, if you click on the video you uploaded and then copy the link on the address bar, it doesn't work. Please do that before the 7-day mark.

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We don't need a video that confirms what we already knew. We can just take this off the unanswered list now.

Answer 1

It appears that I am wrong. Ground-type attacks aren't super effective on pure Fighting-type Pokemon. Below is a shareable link to the file. Please click on it.

https://www.dropbox.com/s/huds1x2yb6vlxti/Video.mp4?dl=0

Comments

Glad the question is resolved but this video hurt my soul xD

Also (spoiler) why didn't Hitmonlee lose 50% health when it missed HJK? Is the damage different in those games?
Okay so Bulbapedia says in Gen 4, HJK crash damage is 50% of target HP, so now it makes sense.