I find that many touchscreen UX problems have more to do with hand-eye alignment than with actual calibration or sensitivity of the touchscreen. This is true for cell phones, tablets, or bigger screens. The smaller the screen, the more acute the problem. We tend to have more fine grained control of our dominant hands and so tend to hit the intended target easier, and be more aware of where our touch is actually landing, than we might be with our non-dominant hand.
These days the vast majority of our touchscreen experience is with a handheld mobile device that we're looking down on and is presented at some angle to the body. Whereas a scope is presented in a perpendicular orientation away from our body, that we have to reach for. It makes logical sense to me that this less-familiar awkwardness combined with the small-ish screen of the DH800/900 screen could challenge a new user trying to use their non-dominant hand. I expect just like with texting on a tiny cellphone keyboard, you get better with practice.