I think it's them. Here everyone seems to trip over themselves to say how their child is doing OK when they're doing pretty well in my book - humility and modesty all the way! I do have a few friends who acknowledge their child is freakishly talented though e.g. those free reading or doing multiplication before school, I also know a few children who were doing brilliant illustrations from a very young age.
Feel awful saying this but DC1 certainly isn't as naturally academic as I was as a child - reading, maths, learning things that came quickly and easily to me and he struggles a fair bit. Maybe if I'd struggled at school, I'd think he was really bright though? Could it be comparative?
Overall I'd say DC1 is average, yes. He isn't the most socially confident child either. BUT like you I agree that's fine. He is naturally very curious, generally kind, a lovely tuneful singer and has a great imagination and sense of humour.
DC2 is only 3 but seems to be a bit brighter/ sparkier than DC1. Excellent speech and mature ideation for his age, great memory, quick-witted and a bit more fiery. But time will tell.
TBH they change so much, I've read so many stories on here where a child has gone from bottom to top of class or vice versa in a matter of years.
Being naturally bright when I was younger didn't do me many favours in the long run either - I'm a praise junkie, if I have to try hard at anything I lose interest and my motivation and sticking power are both poor.