Also it actually kinda is how tiers work.
Every character in the game has the same amount of options, in theory. They all have the same moves, universal tools with invincibility, etc etc
What separates characters often is intrinsic 'extra options' or options which are disgustingly skewed.
A single move on a character could be the straw that breaks the camels back, in both overpowered or bottom tier types of ways. If MK lost tornado potency in Brawl, do you think he'd be #1 still? -MAYBE-, but I would be more surprised if he didn't end up dropping off heavily in tournament results at all levels of play, plus many match ups would skew distinctively away from his favour.
People were quick to claim that Falco with bad lasers was bottom tier; although I think most of Falco's strengths were maintained beyond this and hence he was underrated. It goes both ways.
If Sheik and Greninja both have their projectiles nerfed, then their abilities to combat evenly with zoners is diminished significantly, while their ability to camp out precision characters / force approaches by them are less as well. Stuff like this could be universal -5s in match ups, and that's definitely enough to bump a character a tier or two down.
is a character I've had excessive amounts of play time against in comparison to most. The only Ness in the world who had tournament results for a very very long time was my main training partner. I've played against FOW and Shaky; losing to shaky in tournament (ya ya, I know, Mars losing to Ness?).
ALL THE THINGS I COULD ABUSE AGAINST THIS CHARACTER NO LONGER EXISTS IN SMASH4. His range isn't less than others on his normals (his fair was outdoing everything already), his good grab game in Brawl for combo/legitimate standards was very good and untouched, and now in comparison to other character's grab games, he's a monster.
Back throw as a kill move that's most definitely a kill move (nearly all the other throws with good DI/stage position you can survive upwards of 150%... Ness? No you're going to die at 120%). Kill throws are bonkers in every game, and now no character bar palutena has a comparable grab game. Grabs are overpowered in every game and usually define who is top tier (Diddy down throw anyone?)