Hm I don't think we can unpick this for you from afar, without a lot more information.
Laziness, boredom, perhaps not grasping the concepts as well as you think, has been mentioned as possibilities. But there are more.
My DS is fairly able but in Y1 in maths particularly, he didn't want the teacher to know what he could do (e.g. he tried to hide from her that he knew his 5 times tables). He would make deliberate mistakes, answer 'I don't know' when called up, never volunteer answers in class. I am not entirely sure why he did this - he claimed he hated being on 'blue table' because they constantly had to 'work independently' whereas he preferred group work when there was an adult there. It may have been due to group dynamics / social maturity rather than with academics per se.
His teacher noticed that he picked up new concepts very quickly and easily (err - in some cases he didn't have to 'pick it up', he simply understood it already beforehand, but didn't let on) but never really did stretch him much because he indeed never appeared 'secure' to her in his classroom work. He put some effort into creating that appearance!
I used to coach a young chess player at World Youth Championships and such. At one stage, she lost lots of important points due to 'rushing'. In her case I worked out that it was a kind of perfectionism/fear of failure that was the culprit. She figured it was worse to make a wrong move if you had thought about it a long time, than if you made the same wrong move quickly. In the latter case you made a mistake not because you were not clever enough, but simply because you hadn't given it much thought. Whereas the result is the same - you lose your game and point - it doesn't reflect badly on YOU, on your 'inherent ability'. You and people around you can still think of you as clever.
By refusing to 'try' (properly, giving things the necessary time and effort) you cannot fail. Well that's the perfectionist's response anyway - growth mindset can be the antidote to this. Talking about effort, not results. Discussing what /how much you learned, not how many you got right.
With your child, OP, I am wondering two things:
Clearly your child is learning. The spellings are correct in their writing, which matters more than the spelling tests anyway. Similarly they may not get great scores in maths tests/sets of problems, but they do learn the concepts and are able to apply them successfully in the future. It seems to me that that is not too bad at all - I prefer it to the situation where someone is able to mechanically apply a process to problems and get 100% right, but with little understanding, and once the next process is covered in class, forgets or struggles with previously taught ones.
So what exactly is the problem? Would you like the teacher to recognise your child's ability, in order to a) get good reports, b) access further challenges/stretching? Does your child want these things? Is there a good chance that they would actually be given more interesting, varied problems, sidewise stretching, if their ability was recognised? All too often, the very able are basically given more of the same (e.g. same concept but with larger numbers). I don't think that this would appeal to your child...
- If it were indeed valuable for your child's abilities to be recognised, would your child be amenable, as PP suggested, to an open discussion about this? Where you recognise that your child may sometimes find things boring/may sometimes feel too lazy to apply themselves, but explain that the teacher needs to see x happen before they can give them y.
As long as it doesn't mean your child is missing out on some amazing opportunity, IMO at primary age, and as long as you are certain that your child is secure in the concepts, and CAN apply them when it counts, I personally wouldn't worry too much. What is your priority - having them at the top of the top table (which no-one will care about in the future) or having them really properly comprehend the concepts (which will make a massive difference in the future).
Perhaps provide them with some sidewise stretching at home - coding, chess, NRICH, Wild Maths come to mind.