It happens more often than it doesn't. Someone looks like an NBA player and plays like an NBA player, meets all the prerequisites. There's the size and speed and athletic ability, the wow dunks and know how, the special prep school, the McDonald's and Brand Jordan game, the right university of higher scoring and coach whose been an NBA gateway, and then it just doesn't happen like it was supposed to.
The difference, often, is negligible. It seems sometimes you can slip a piece of onion skin between the guy who is a celebrated first round draft pick and the one who looks and seems just like him who isn't, who is begging for a look at Summer League, dragging through all those nights in Canton and Stockton and Grand Rapids and out of the rotation in NBA places that are out of the rotation themselves.