I'm thinking your DC has always found everything easy, no real effort required to get top marks, they just knew it all instantly. So they expected this to be exactly the same, but it isn't. There are people in the country who have got higher grades, maybe even in the same class (and who the DC has always thought weren't quite as able as them).
The reality check that actually, you're not that perfect is challenging the way they have learned to see themselves, maybe reinforced by school, their friends, other kids and possibly by you. And it's not fun realising it.
Logically, being in receipt of all A, A star and A star star (if you look at it in old money terms) is a great achievement. But it's not showing practically perfect in every way and clearly the smartest in any room, so it's going to hurt, even for the more pragmatic kid who recognises the equivalencies (and isn't quite so demonstrably emotional about it).
I'd give them the opportunity once they've calmed down a bit to still go for the meal (hunger and lack of sleep could be contributing) and if they're not up for it, a giant tub of icecream, cookies and silly sprinkles later this evening might be a way of softening them up a bit.