Welcome, Oakie!
I do hope you stick around. There are a lot of really helpful and knowlegeable people here and it sounds like you belong.
I just wanted to reassure you that, to my knowledge, this is very common among gifted kids. When you don't have to fully engage your mind to complete a task, you're obviously much more liable to make careless mistakes. Harder work requires more attention and usually results in a gifted child making fewer mistakes, not more.
I remember consistenly scoring very well on high-level math problems on all the standardized exams and getting high scores, but when you looked at the breakdown, my lowest scores were always on the basic computation sections, and not because I couldn't add and subtract! I've seen this with my kids and their school work as well. And while I understand it and am okay with it (really, I am!) it can be frustrating when you're asking for more academic challenge for your child and they keep pointing to the silly mistakes they make as an excuse for not offering the proper challenge.
Hope this helps!
