The butt, which comes from the top of the front shoulder, is fattier than the loin which is lean and comes from the top of the back along the backbone.
The butt is meant to be cooked as you did, low and slow to break down the connective tissues and render the fat. This is the cut used for pulled pork.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan