I wonder, too, if you've ever spent any time around more typical kids. With my DD, I was worried sick she wouldn't test gifted, only because it would have hurt her feelings because of her older brothers. She did not seem as obviously gifted as my boys. One, she had a speech impediment and two, her vocabulary has never been as advanced (ironically--now I realize DS vocab was really more a function of his 2e than his IQ).
Other people would always say--WHOA, your dd is something else. And I'd be like, she is?
She had the same GAI as her obviously/stereotypically gifted brothers, turned out. I think it is harder to identify when you have a certain model in mind, maybe. I used to think other children might actually have something wrong (delays) when they were really bright children, because of my limited exposure to "normal."