According to smogon users, the first factor that divides tiers up and the most important is how many people use them. Not on Wi-Fi though, oh no. Just on Shoddy Battle, a battling simluation program where people test teams and battle for fun. This means that only the more intellectual battlers use it and people who use Pokémon such as Pikachu do not exist, thus making Pokémon like Pikachu NU dispite the fact it is used in almost every kids Platinum team. This is a problem in itself.
Because pikachu has god awful stats and folds after an Earthquake from OU? By the way, in game teams don't really matter since everyone in game apparently got hit in the head repeatedly. That is why they make their Rattata keep using growl when you have 1 HP left.
The next factor, according to smogon users is movepool and stats. Now this is completely wrong because there are plenty of NU Pokémon with stats equal or better than that of an OU Pokémon. Lets compare one, shall we?
Let us. You treat stats like it is the only thing that matters in the game. "My Aggron can beat your surfing starmie down to a pulp because it has 10 more BST AND ROCK HEAD!"
Gardevoir Statistics:
HP: 68
Attack: 65
Defense: 65
Special Attack: 125
Special Defense: 115
Speed: 80
These stats seem pretty average for a UU Pokémon right? Exactly, but that's not all. Gardevoir has an excellent movepool which includes great attacks such as Psychic, Energyball, Focusblast, Thunderbolt, Hidden Power, Grassknot, Shadowball, Will-O-Wisp, Wish, Hypnosis and many other useful moves. That's not all! Gardevoir has the unique ability Trace which copies the opponents ability.
See what happens when you use that against a Slaking, the one hit pony. And even then, many more pokemon can use the moves that Gardevoir has, except better.
This means Gardevoir can switch in on Jolteon and Vaporeon, two top tier OU Pokémon and basically laugh in their faces. Guess what? This Pokémon is NU.
So what? Gardevoir will crash after a physical dark type move. Pursuit, feint attack, and other moves can beat this pokemon to a pulp when stat increasing moves are used correctly.
Alakazam Statistics:
Hp: 55
Attack: 50
Defense: 45
Special Attack: 135
Special Defense: 85
Speed: 120
This Pokémon is always coupled with Syncronize or Inner Focus. Both of which are pretty much terrible abilities compared to Gardevoirs rare Trace ability.
trace is completely circumstantial. Not all pokemon have good abilities that can be used instantly, you know?
Alakazam has a much more limited movepool too, the only special sweeping moves it can learn of any value are Psychic, Hidden Power, Focus Blast, Signal Beam and Shadow Ball. These moves are pretty typical and can be predicted. Sure Alakazam has slightly better special attack and hugely better speed but Gardevoir makes up with it by being much more bulky and having that rare ability.
Bulky? LOL, 65 base defense is bulky? The max defense it can get(including 252 EV's, beneficial nature, AND perfect IV's. Granted, Alakazam has worse defenses, but the fact remains that gardevoir is not bulky. Just see how long it lasts against Crunch Tyranitar or an average Heatran.
This Pokémon is OU which is common knowledge but comparing it to Gardevoir close up makes it seem pretty pathetic in comparison.
There are plenty of other Neverused Pokémon which aren't bad by any means, one of these is Ramparados which has one of the biggest attack stats in the game. (165) This is bigger than any Rhyperior, Jirachi or Aerodactyl.
Rampardos is a glass cannon. Although he can deliver a lot of damage to the opponents team, he will most likely not have the chance to do so. In case you forgot, Surf is common among pokemon. And with pokemon like Starmie running around in OU, it will not matter much how Rampardos CAN dominate if he is dead. With choice scarf I could see that Pokémon tearing teams apart in OU. (Plus it has an excellent movepool) Another example is Aggron with a monstrous 180 defense and a respectable 110 attack stat. It also has a great movepool and a pretty acceptable ability yet it is NU.
LOL, like before, Aggron is not a great pokemon to use. He has 60 base Sp. defense, and 50 base speed to boot. Speed makes a huge difference in games, and you are clearly undermining that. And he has few toys that he can use to bar his weaknesses. 4x weakness to ground won't help that pokemon when Earthquake is one of the most common moves in the whole game. And a focus punch breloom will make child's play of this, if my psypokes calculator is working correctly of course.
Looking at all of these NU Pokémon which are really worthy of OU makes me sick, if you use a Pokémon like Gardevoir in the OU tier on shoddy then you're liable to be laughed at for using an NU Pokémon. Nobody gives these Pokémon a chance, they just stick to the same overused Pokémon in every single Wi-Fi battle.
Then why don't you make a difference? See how far you can get with Gardevoir in OU. You will be walled by Weavile with no effort whatsoever.
Tl;dr - Smogon is wrong, they're tiering Pokémon up based on the same Pokémon that are used over and over again. All people do now is grab the first set they see on Smogon and throw together a pretty darn predictable team. This can be seen on Serebii Wi-Fi chat, check it out there.
Smogon isn't the honcho of everything Wi-Fi in pokemon battling, but it is known by many people as the most reliable source. Anyone will admit that Smogon is fallible.