Well.. It happens with any competitive game, and it depends on what level of play you're looking at. Every character has strengths and weaknesses, and certain techniques only they can do. Obvious weaknesses like Ness/Lucas having to use PKThunder as their recovery are easy to notice. Weaknesses like the infinite grab against Ness and Lucas with Marth are found out as the game is played. Low level players wouldn't be aware of things like hitboxes, priorities, and advanced techniques, so tiers don't matter. High level players know all about these things and how to exploit them, which is why tiers make the most sense at this level.