Don't get too hung up over the letters. They don't mean what they usually mean, and this is why it's important to read the notes to get an understanding of what and how EXACTLY they are ranking these characters and what the purpose is.
For example, jiggs is in D tier, which usually means really bad, but she's actually around the middle of the cast.
In SHI-G's list, the letters are abritrary. In smash 4's case, they are very misleading, and really imply the game balance is much worse than it is. If we were to compare Smash 4 to Brawl, Smash 4 is at WORST the equivalence of the cast ranging from a low of characters like Ike, Shiek, Yoshi, (the D Tier in Smashboard's tier list) to a high of the A- tier (Marth, Snake, Falco). If there aren't any more patches, this is the worst it's getting. Personally I feel the balance right now is the equivalence of D Tier to B Tier (Pikachu, ZSS, Wario).