Recognising aptitude in a certain area of study can be mistaken for arrogance. To avoid this, use of overt modesty is advisable.
Self-explaintory really. Even just saying, "Oh here, let me do that. I'm good at it" can be seen as over-confidence, which I think is silly. I mean, if you're better at something than average and you enjoy it, why shouldn't you be able to say, "Yes, I can do that and I can that well"? I don't know, it might just be a British thing and nothing to do with autism at all.
This is one of those things that I was told about straight after doing it. I vividly remember being in Year 7 or 8 and overhearing people talking about how arrogant I was for answering all the questions posed to us in Maths and putting myself forward for doing the Maths Challenge. I enjoyed it and was incredibly good at it and I was genuinely shocked that other people were bothered by lack of humility. Now-a-days I just feel ashamed if I point out my skills and I don't think that's right. We should all be allowed to be proud of things that we're good at.