Just a few observations..
Sometimes even though a piece of meat is smaller in weight, the thickness is the same as a heavier piece and as I have mentioned before, the thickness of the meat plays a large part since the heat must reach the center in order to cook the meat to a finished temperature.
When I purchase briskets, for instance, on the small side in order to cut down my cooking time, I am looking at the thickness as well as the weight.
Even though this was a 1/2 butt, it looks pretty thick to me from your pictures.
I also noticed that you took a picture every hour.. this is great for logging your project however, the UDS as well as other smokers known for long, steady cook times do much better when all things are left alone as much as possible.
Unless you had a camera mounted under the lid, every time you removed the lid to take a shot it probably added a bit of time to your total.
Other than that, I always tell folks to let the temperature tell you when the meat is finished rather than the time. I have had some really funky unexplainable things happen while cooking low and slow.
Try to estimate but always be prepared to stick it out for as long as it takes and/or have an empty cooler ready to place it in to keep it hot if it gets done early.
The real question is.. was it tasty? That is the measurement of success around here
