Yes there might be a background that is more fitting for this character. I just pulled something out of my portable hole. I wasn't thinking too hard about the specifics :P
Maybe I should have said that my character is an inter-dimensional ferry runner and so it makes sense to get +2 to knowledge (planes). Maybe they run a ferry courier service between two hostile nations so it makes sense for them to have +2 Diplomacy instead. And on and on and on... It's easy to come up with backgrounds that don't fit a generic mold at all.
My example was to illustrate a general problem: No matter how extensive the list of stories and benefits to match, if those stories and their associated benefits are inflexibly attached to each other then there will always be large gaps in the stories people want to tell about their characters and the mechanic that underlies those stories.
Yeah, this is actually a good point. Hopefully every background will include some floating stuff, with some thematic suggestions for how they could be applied.
So maybe the Merchant has +2 Cha +2 float, and grants Diplomacy and a floating skill. Then has some suggestions for how those choices can be flavored as examples, such a suggestion that maybe picking Medicine as your Merchant's floating skill might mean you have a background as an apothecary, or picking Society might mean they are a courier or supply the masquerade craze among the Nobility of (insert city here).
I think providing examples like that in the book is useful, and can help get players' minds and imaginations going. As long as they are clear at the start of the Backgrounds chapter that the suggestions are just that - suggestions. :)