We went through something similar with our DS (now 8) and I know of another gifted kid who's the same age who's mom had similar issues back in JK/SK.

The root causes turned out to be different but here's some potential explanations and maybe these will inspire some questions for you depending on which ones ring a bell -

- some teachers don't give out top marks early in the year, they want to show development and leave room for improvement and growth. On the flip side teachers can often downplay things that they think the kid will outgrow but that actually are issues which makes it challenging.
- engagement in the class - some kids lose interest when the level is way below them. If the class is doing "this is a P, it makes a pew sound" you could see how a kid reading chapter books might not be the first to contribute to class discussions.
- personality - DS in particular was really shy and didn't speak for most of JK/SK so how were they to know what was going on in his head.
- teacher fit - one of DS's JK/SK teachers actually told me that she just butted heads with DS and they didn't get along. My jaw dropped to the floor that a teacher was telling this to my face about a 5 year old but it did make me wonder how objective her grading was. The feelings went both ways, DS wasn't exactly showing her his best side either.
- they might not test above level - My DD often came home with report cards that had ridiculous things like "can count to 20" when I knew she was capably of much more. They just only asked to go to 20 because that was the check box they had to cover.
- Learning Disability (LD) - we discovered that our DS is LD which explained a lot of our particular school issues. The other child has no LD issues so don't panic just yet. Just mentioning it because it might be something to be aware of (I know it wasn't something we were looking for or expecting). In our case it became more of an issue when writing was starting to be used in evaluating but there are many different flavors of LDs that have very different impacts.

I was able to volunteer in his class and it was a very eyeopening experience. If there is a way to volunteer or observe for an hour I'd highly recommend trying.

Hope something in that inspires something. Good luck!