So to my mind and experience, talking about sporting achievement is an everyday, possibly irritating to others, but not friend loosing, variety of potential bragging. The costs for ultimate success are high and so that talent is only a threat to the person willing to make similar sacrifices - and the benefits of success are shared. Talking about academic achievement effects me personally. It means your child might have better life chances than mine (because the majority of us are competing for jobs in what is theoretically a meritocracy), I might feel that I should have done more to make little Sarah smarter (believing I could have influenced her IQ), I might feel that your child might be tricksy and manipulative - might out smart and take advantage without deserving it. Your child might be getting access to more resources than mine, further increasing their advantage etc, etc.
Of course I am making generalizations here and I am not suggesting these are conscious thoughts. I just think intelligence is a much more loaded advantage than physical skill.
I don't agree that the benefits of athletic achievement are more "shared" than the benefits of intellectual achievement. If your friend's daughter trains hard and improves her swimming time by 1 second, who benefits? If your daughter studies hard and becomes a good doctor, many people benefit.
