It depends on what you're thinking.
If it's "he's not in the top group", then it may be that there just are children that are better in that form. Doesn't necessarily mean he's worse than you though, simply others are better. For example I used to help when dd1 was in year 1. I often took group 1 and 2 (separately) for a maths exercise, then took group 1 from the parallel form. The group 1 from the parallel group was nowhere near as good as group 2, in fact I think there was only one child that would have stood a chance of being in group 2.
It may be that you have overestimated how good something was. I've remember a parent telling me she was going to complain about the health visitor who had made a
look at the 2 year check. HV asked if child said 20 words (minimum for intervention). Parent responded with "I think they must be a genius. I counted yesterday and they have 120".
It can be that the child is not producing at school in a classroom environment what they produce at home 1-2-1 with you. That is perfectly normal, in fact more common than otherwise. Think about it-even for me I can produce much better and quicker when I'm on my own at home than when surrounded by demands at work.
It may be that the teacher knows they can do it, but feels they need the confidence boost of repeating easier work.
Or that they are not ready for the next stage.
Or they're still assessing them and they'll move on very quickly.
Teachers do sometimes get it wrong, but in many years as a parent I have very rarely (once I think) heard a parent complaining the teacher was overestimating their child. You would expect them to get it wrong both ways fairly equally.
And in cases where I have known enough about the child to comment, the parent has always been overestimating the child's ability