KLF,
Yes, tenderloin is very different due to the low fat content. 190-200 for shoulder is used because the collagen and connective tissue begins to break down about 185-ish. Aim lower if you want to slice the meat instead of pulling it. 160 is still technically "done" but it's a texture thing with this cut.
Ken,
Every piece of meat is different, it could just be that. I was thinking that it could also have been the shape. Shoulders are kinda strange sometimes and you might get a sloppy cut that lays kind of flat. That will cook faster than one in a tight, compact shape. Use butchers twine to tie a sloppy cut into a tight, uniform package - it will cook more consistently and take longer to temp.
Also, I'm not familiar with your cooker (I tried to look it up but the internet was busy!) but where on the rack you place the meat in relation to where the heat is or enters the cooker body can have a big impact on cook time. Even on cookers with an offset smoke/heat chamber. You might just have to go to a lower temp if you want a longer cook time since you say it happens on other meats, too. But the end result is all that matters. Keep us posted.
--john