Well, I cannot figure out why it is impossible to get more tha 96 percent. There are 2 possibilities:
1)It is possible make 100%, but nobody is able to get them :)
2)It's a graet trick gamedev, to make us play again & again.
P.S: It is somehow strange that you gan get all the coins only once. If have got 5 coins, all specialcoins in the level in this area disapear, until you reset the game.