Max CP for all Generations
Who the hell predicted Rayquaza to have a lower CP than Groudon or Kyogre? https://pokemongo.gamepress.gg/max-cp
It is a well know fact that Rayquaza is the strongest Pokémon ever created by Arceus.
Answers
If you're actually serious, the predicted CPs are based on how CP is currently calculated, which uses the base stats from the main game, so unless the current CP formula is changed, or how the main series stats get translated to Pokemon Go's stats, that is the CP that Rayquaza will have if it gets released.
Well, it comes down to the current way of calculating the attack and defense stats of each Mon since late November. I suggest you read this to understand what changed then : https://pokemongo.gamepress.gg/pokemon-stat-change
To sum it up, "specialist" Pokemon recieved a huge boost thanks to the new formula, pulling Rhydon, Alakazam and others out of the shadows. What I mean by "specialist" is a Pokemon which, in the main games, has one of the physical/special stat higher than the other. PGo "simplified" the physical/special split by adding more weight to the highest of the Attack/Defense stats.
The main difference between Groudon/Kyogre and Rayquaza is that G/K have one very high defensive stat (bases 140/90 for Groudon). Rayquaza has two huge attacking stats, but its defensive prowess is average at best (90/90).
This led to Rayquaza having about 2/3 of the other titans' defenses in Go, hence the difference in CP.