Hmm, I admit I did think dd was bright at 10, but her primary school last report very much focusses on her writing of stories, and reading age etc (same for dd2) - and her maths had to be given a determined push in year 6 to make sure it didn't let her down in year 7.
When I bumped into her old teachers again after she'd left, they would say 'oh is she still writing lots/enjoying reading?' etc, and that was definitely her strength until a little while into secondary, where she found she actually enjoyed and was good at maths, rather than feeling that, as a reasonably bright child, she ought to be good at maths too.
I think it's partly that maths is more interesting the further along you get with it - both mine were a bit freaked out by times tables and 'grids' at primary, but dd1 actually enthuses about how much she likes differentiation (I don't know what that is in a Maths sense, but she likes it).
So anyway - I think had she been sitting an 11+, maths would have caused us some stress, and if anything had let her down, it would have been that. So I am pleasantly surprised and consequently cautious about deciding a child's strengths and weaknesses at 10.