I have no clue how they're determined but I'm pretty sure it has to do with being in the same league(rank), considering I have got my deck to 3400 might and gives me a good boost of health and the other Adam abilities over other opponents I've played against. I have +140 health the other guy will have probably +/-120 health.
Those guys are from an event that came up about 1-2 months ago. Considering they were ranked at 19/20 they probably bought them in the event bundle and then used the event currency to buy the event chests. You can get them legit from the event journey but it's hard as hell, I played that same event as a R-25 and only got to the second page of the journey. Word of the wise, if you see some players that aren't the high ranked and have some serious firepower, they probably spent some money on the game. Just the other day I saw a guy in the game chat that was a R19 and his deck had the firepower that of a R13 deck, I'm a R16 and this guy could've wiped the floor with me... It was pretty obvious he spent money on the game.
Even a R14 asked how much he spent lol