Yes...the discrepancy in cooking time is between the Boston (marbled fatty goodness) and picnic (leaner with pockets of fat) parts of the shoulder.
Boston is marbled fatty goodness, and no one has any problem with that.
Picnic is leaner with pockets of fat. It's pork so it's good, but it's more erratic in terms of fat distribution and can be touchy to cook.
I have seen this thing most clearly with a bone-in shoulder cut in two.
Fatty half will be done in 10hrs, perfect, resting comfortably.
Lean half will be just hitting the stall at 9hrs...seems real firm like it's not gonna end well.
The easiest solution if you're cooking a whole shoulder is to wrap the whole shoulder in foil...when the butt (fatty guy) hits its stall temp of 165 deg F or so.
When the wrapped pork hits 190 deg F (measured in the Boston Butt side of things), you can unwrap and return to smoker until it hits 200 deg F to get some of the bark texture back.
There's always a balance with brisket and shoulder between bark and moistness, but you can get both if you work it out.
Note also, re moistness...resting after the cook is big. Totally recommend a wrap. I do foil with layer of Saran wrap on outside for pork, butcher paper for brisket). I do at LEAST an hour sit to get the juice redistributed--often much longer. Not amazing for bark, but worth it for the meat in general.
Also...if you still have some dry meat..pull the dry and mix with the more moist stuff. Should help.
I also do a bunch of spritzing with apple cider vinegar mixed with apple juice and a little rub dusting once pulled, to get it just a little extra flavor--sometimes the most "internal" pork will be a bit plain otherwise.