Let's explain.

Thor aka "The Mountain" from 'GoT' deadlifted 1,104 lbs. on Saturday ... breaking Hall's previous world record of 1,102 lbs.

Hall -- the 2017 World's Strongest Man winner -- was the first human being to break the 1,100-pound barrier in 2016 ... a lift many thought was impossible.

So, what's the issue??

Well, Eddie set his record at Giants Live World Deadlift Championships -- an official meet -- complete with other competitors and a packed room of rowdy fans.

Thor's record -- 'cause of the global coronavirus pandemic -- was done at his home gym ... which Eddie claims makes the lift and record, bogus.

We talked to Bjornsson ... and uhh, let's just say it's clap back szn.

"I honestly just think that [Eddie] is trying to protect the thing that he had. For him, that meant a lot to him to have [1,102 lbs.] world record," Thor tells us.

"He's just trying everything he can to make people believe that my record doesn't count. But, it does count. It is a world record and that's just how it is."

Contrary to Hall's claim, Bjornsson says the home gym conditions actually made the lift wayyy harder ... not easier.

"With my experience, and I've been in the sport more than a decade. When you can compete with the crowd, that gives you an adrenaline rush, and gives you the extra strength you need to perform better."

"Doing it with just a referee, camera crew," Thor says ... "It doesn't feel the same."

Any way you look at it, the lift was crazy impressive ... but Thor says he could've pulled significantly more if he wanted to truly push himself.

Just how much more?

"I can sit here and say that I could of pulled comfortably [1,124 pounds]. That's just me being completely honest. I do believe that I could have pulled possibly, with a grind, about [1,146 pounds]."

BTW, if you dig these 2 giants -- former-friends-turned-enemies -- going at it ... they're boxing in Vegas next year ... and The Mountain says he's ALREADY training to beat Eddie's ass.