Hey there,
After checking some of the calculations of the scenarios that you posted, I did some research and unfortunately couldn't verify the two situations you posted about.
(Do keep in mind that these ev values are actually LOWER than what bosses actually have and therefore your attacks will actually be doing less than what is posted here)
The first being Haxorus vs Meganium:
+6 252+ Atk Mold Breaker Haxorus Dragon Claw vs. 252 HP / 252+ Def Meganium: 435-513 (119.5 - 140.9%) -- guaranteed OHKO
Now, this stat may suggest that your Haxorus could knock Meganium out in one shot, however Elm's Meganium also knows Reflect. If you are unaware, Reflect raises the defense of the user's team which then changes the calculation to this:
+6 252+ Atk Mold Breaker Haxorus Dragon Claw vs. 252 HP / 252+ Def Meganium through Reflect: 217-256 (59.6 - 70.3%) -- guaranteed 2HKO after Leftovers recovery
This reading seems closer to what you have suggested happened. Since you stack Dragon Dance 6 times it seems likely that Reflect could have been used by Elm.
The next scenario you posted is calculated below:
+6 252+ Atk Mold Breaker Haxorus Slash vs. 252 HP / 252 Def Togekiss: 289-340 (77.2 - 90.9%) -- guaranteed 2HKO after Leftovers recovery
Even if Reflect wasn't up, there is no way that Slash will ever deal enough damage to defeat the Togekiss in one attack (unless of course it lands a critical hit).
If you could record evidence of this happening then I would be happy to look into it for you. Specifically, if you could record what pokemon you are using, ones that the opponent are using, as well as the moves that they are using during battle.
Hope this has been helpful! If you have any other questions or concerns please don't hesitate to ask.
All the best,
Gouda