Corrosion on bolt thread and cone. The real solution to prevent this is to put a dab of grease on the thread before you tighten it up. Even if it is dried grease, it work.
Surefire way to knock the lug off is to use the factory lugwrench... put it on so it is at 9 o clock or lower position... make sure it is inserted properly... and use your foot to strike ot loose.
Next step up is 6 point socket with flex breaker bar... 18" is all you need... grab it with both arm and use your upper body weight to push it down... up down up down....
Now if you rounded it off... it is because you used 12 point socket... find next size smaller SAE socket and whack it on and see if you can loosen it up. Otherwise, you are gonna have to get a socket that grabs the middle of the nut to get it out. Harbor Freight used to see this.
I don't care what impact gun you got at 300 lb/ft is nothing. My Earthquake (harbor freight) has 600 lb/ft reverse and it sometime struggles but does get it out. Breaker bar will get out whatever impact gun cannot do... pure and simple.