Actually, there is a certain thing I've thought of:
Noland's Articuno, despite being a Legendary, lost to Ash's Charizard.
The Articuno in The Power of One, however, was extremely powerful, if I recall.
The Palkia that I caught on Mt. Coronet seems much stronger than the very same one that lost to a Roserade on Pokemon Battle Revolution.
I ask you this, is it possible that a Legendary Pokemon becomes weaker when captured?
And the regular Pokemon, the equilibriam theory is suffice. A regular Pokemon -if given a fair match, such as Dragonite vs Snorlax- is balanced against another regular Pokemon. Thus, that is why everyone in Pokemon World isn't dead.