I'd agree that having a younger child outperform an older at least in some area is a fairly normal experience even when you are not talking actual giftedness. It calls for tactful handling- but then parenting does. Most larger families will have a younger child who is good at something, whether sports, music or academic work. This is one of the benefits of a large family: you learn to handle other people's cleverness.
This is not to deny that there are children who are so gifted they are just off the scale and this certainly creates problems of its own in family dynamics.
Also I agree that it is very much a personality question whether you hide what you can do or not. It is not something that is directly proportionate to your level of giftedness.
My ds is desperately into hiding what he can do. But there is no sign that he is gifted; in fact, I'd say he is of fairly average ability- if only he would show what he knows. But he is so frightened of finding out that he can't do well even if he tries that he never does try. In his case, this is an atittude brought on by finding that his friends were more gifted than him, and that things that are quite difficult for him are easy for other people.
Dd otoh, who is gifted, doesn't seem to worry about what people will think about it: she seems more easily convinced that people will like her because of other, totally unrelated things, and that giftedness just isn't an important part of the equation. To her, likeability is about taking an interest in other people, and her ability to analyse poetry or read adult books on Renaissance history doesn't come into that. She can do both.
I was possibly gifted (I certainly had no problems in outperforming my 4 years older brother) and I was bullied for it, but it only made me more determined to push on and achieve as much as I could because my bullies were wrong. Likeability wasn't something I was interested in.
So totally different reactions.