I think it's more like he has a match-up set that people would kill for, but his worst match-ups are the commoners of the tournament scene and he isn't really exercising his stronger advantages nearly as often against someone like Pikachu, Kirby, etc.
At least, that's my own little theory. 60:40 can make fights tougher for Marth's opponents, but they're all perfectly winnable fights, and Marth has tons of those. Meanwhile, you have characters that just completely take out others, whereas Marth takes out characters who are already covered while still falling sometimes to the top tiers.