I'm assuming you're child A's parent.
I think it depends. "Traditionally" what actually does that mean? Probably? Usually? It's not a rule, and the school probably doesn't have that rule. It's the parents that assume. It's probably that most of the time they tend to give it to the top scorer unless someone else stands out with effort and potential.
We had a few of those at primary, but they also had the rule that no one got more than one main prize, and sometimes they felt someone deserved it more than the traditional winner.
By traditional winner, I was down for 3 prizes. I got a 4th. I remember dm being a bit indignant for me. However I was more chuffed to get the unexpected one (was citizenship-for being "consistently helpful") than I would have been to get the other 3.
Our academic cup more often than not went to the top in the end of year tests. However I can remember it going one year to a boy who had arrived in the juniors not able to read and with no idea what a number even was. In the year 6 tests he came 10th. The school said that he'd put such effort in to prove that he was able to do these things. Half his year cried with him when he got it.
I also remember the music cup going to someone who didn't expect it in my year. He'd never won anything through school, was bottom of the class, non-sporty, not even very popular. He'd sung a solo (very nicely) with the choir. No one would have given his name as a possible. But he got it, and judging by his face he was over the moon. I remember feeling as a year 6 how nice it was for him to get it even though I wasn't friendly with him.
The sports' day thing is odd. But I'm wondering if they won all the races whether there's a bit more to it than that. I've never seen a situation where one child wins all-they usually have ones they're 2nd or 3rd. If that's been the case every year, I wonder if the staff saw the rest of the year getting dispirited and thought they'd award it to the second too so there does feel to be an element of competition and worth trying.