Relative power versus Universal power is something that isn't always apparent or easy to gauge. Especially not just from simple observation.
For example, a character could be improved in 2 ways
- Give them a move or cancel properties that is useful/advantageous in all matchups.
- Give them a move that specifically counters another character's move.
To elaborate, giving a character an invulnerable DP is a universal tool that can be used against just about any other character. Likewise, not giving a character any reversal-type move (or any inv. move) universally handicaps this character relative to to the rest of the cast. What this means is that we could see a character moving up or down in their tiers just based solely on these qualities, which is why many Western players feel that if a character has a apparently overpowered move then they must be top tier.
On the other hand, there's sometimes ways to improve a character within the tiers by giving them moves that counter a specific other character in the cast. A broad example of this is like giving a character a throw-invulnerable move, which would be significantly more useful against the grappler-type character(s) in the roster than the poke/runaway type characters. A more specific example would be to give a character a move that dodges either just high enough or just low enough to beat a specific character's pokes, but do not have that property against other characters.
One really good example I'm fond of is GGXX#R Eddie and Anji. Wherein, Eddie was undeniably top tier and certainly overpowered compared to the rest of the cast, and Anji was undeniably bottom tier and very underpowered. Yet, because Anji had auto-blocking on a lot of his normal moves, Anji turned out to have a decent/even matchup against Eddie. The inclusion of autoguard moves sounds like a universal ability, but in this special case it turned out being significantly more useful against one specific character rather than the whole cast because it specifically countered all of Eddie's strong points, which meant that Anji could shut down Eddie's whole offensive game.
Now, while Eddie was still top tier and Anji was still bottom tier, the tiers don't always go unaffected.
A theoretical example would be to introduce half of a roster of characters, wherein character "A" is top tier because of their superior low pokes. However, after introducing the other half of the roster it's found out that these characters are all low tier, but they all have moves that counter low pokes, making character "A" significantly weak against half the cast, dropping that character down from top tier. Even though character "A" seemed to have universally good moves, if half the roster counters this character then you can't rightfully say this character is top tier, even if that half of the roster is universally weak (except against character "A").
I believe this sort of thing has happened in games like Vampire Savior, Garou, and Jojo's where certain characters would be the dominating top tier if not that they have some critical bad matchups which lowers their over all ranking.
And, it's important to keep that sort of thing in mind when evaluating tiers. You can not just look at a character and say that they are top tier because they have a single awesome attack. In a match, you're doing more than one move, so one good move alone isn't enough. You must also analyze matchups, because if you don't then it's possible a seemingly strong character is actually not that good because of bad matchups.
Personally I think tournament results should also be cataloged and analyzed and weighted with other data to determine actual tiers. Reason being, even if everything looks right on paper it may not actually be plausible in reality due to execution. Somethings the easy-mode characters that aren't especially strong do better than they theoretically should simply because they are reliable and easy to use.
Once again, for those that "don't believe in tiers"; it's true that a player's skill is going to be a stronger determining factor in the outcome of a match than character-ability. BUT, it's still technically a potential handicap. Just like turning up/down stars for damage/life handicap in versus mode, picking a character with a bad matchup will also be a handicap. That isn't to say the better player can't win, it's just that it will be much harder to do so.