Up until grade 3 this sounds similar to my DS10. Occasionally he'd get a higher mark in science or math (his passions) but mostly B's and occasional C's. He's 2e (dysgraphia) so we weren't really shocked about low writing marks because he wasn't really being properly accommodated. We were working on trying to improve that as well as trying to get some differentiation in math/science. Careless mistakes happened way more when things were really easy for him (but try explaining that to a teacher without sounding like a crazy person trying to protect a precious snowflake of a kid....).

Where things differ is that DS reads constantly (and way above grade level so we knew that wasn't an issue). He HATED school pretty much from the start and had little desire to please the teachers (with 1 exception - interestingly enough she was the only teacher out of 15 that said nice things about him).

Last year (grade 4) he changed schools into a full time gifted program where he's taken off. He LOVES his teachers and classmates and actually looks forward to school. The depth that he craves is finally there and he is finally in an environment where he can talk to people that can follow his conversation and actually contribute. He is no longer told to be quiet in classroom conversations. Kids no longer look at him like he's alien. His teachers actually get 2e and he is getting much better at typing so he's able to express his ideas on paper/screen. His first report card was almost all A's, his year end was straight A's. We were shocked.

I realize that we are very lucky and that not everyone has the luxury of a gifted program that actually fits their child so YMMV but I do think that school environment is a big thing. I wouldn't go as far as saying that his past teachers and peers bullied him but there were several instances that were far from ideal. DS figured this all out pretty quickly and this resulted in him shutting down and/or becoming rather prickly to deal with.

I also want to say that my experience with 2e has been an adventure of trying to tease out the can'ts from the won'ts. Even with a firm diagnosis (he's been through two full founds of ed psych assessments) there are nights were he's fought me and I can't tell if it is
a) too easy because of the gifted part
b) too hard because of the LD part
c) homework that is taking time away from his current novel/lego project/whatever
d) he's just cranky for some other reason

In my experience kids really, really want to do well and when they can't they are very good at convincing people that they just don't care. If they aren't there are usually pretty good reasons. Unfortunately they can be a challenge to figure out.

Not sure if any of those ramblings help but I can definitely empathize. Hopefully others will have more helpful input.