Giftodd - I am no expert and your theory makes sense. But I also think that even kids with no attentional issues still have more trouble paying attention in a classrooom environment. I was actually just chatting to a friend about this yesterday. She had just done reading with her son's class for the first time since he started at a new school (last year). Her own son has inattentive ADHD but she was commenting that she was astonished that ANYONE managed to learn anything in a classroom and no wonder homeschoolers comment on how little time you actually need to do "work" to keep up with a normal school pace with all that time lost to 20 children trying to find their stuff, sort out their issues, causing disruptions, etc. And this was at a great school with really nicely behaved kids.
I certainly know that my own "normal" DD has vastly inferior handwriting, drawing, etc at school and I don't think it's that she is hiding her abilities. I think it's just that she's not yet 5 and at home she finds it much easier to focus on her handwriting but in a classroom her senses are constantly bombarded with a million other interesting things. She can make herself focus and do what is expected but she's not going to produce the same level of work she might at home... One hopes this improves with time and some of those skills becoming completely second nature.
