slammie, don't start doubting your DD, but if math is not her strongest area, and she is already in a gifted school - she might actually be OK as placed for a little while.
First, most 6-year-olds are not very test-savvy. I think your test results with a 6-year-old will depend on many factors other than a child's ability - one-on-one testing is probably best, but hungry, tired, anxious, or distracted - can throw it all off.
DD9 DYS wasn't all that interested in math at age 6. Her passions were reading and science. She also did not enjoy memorizing math facts (WM is not her strength, either). She always made it into the top math group, but I never thought of her math skills as particularly impressive. Some time last year she really started making some leaps in math, and is now more clearly one of the top math students in her rather large grade (and possibly very high a couple of grades up - which I honestly find amazing, since I know other students spend much more time on math than DD does), but it is still not her favorite subject. So I guess the moral of the story is - give her some time.
BTW, my DD was sometimes impulsive, immature, and bossy as a 1st grader...I was almost more worried about that, than I was about her seeming quite bright (and to be fair, she hadn't been tested yet - so I did not know what you know about your DD). She has fortunately matured, but she had quite a bit to work on other than just academic skills at that point.
What is it with 6-year-olds and refusing to write things down??? Ugh - just mentioned this in another thread - DS6 recently took the WIAT, and would not write things down. Not even when he was told he could. Somehow, he still did incredibly well, but playing human calculator is eventually going to cause him to make errors. Still - they are SIX!!! Also, next to a mathy boy, like my DS - my uber-verbal 6-year-old DD would have looked like a math slug at age 6. It's not that she didn't have the ability, but next to a child more interested in math - she simply didn't have the knowledge that comes with applying oneself in math (that seems to be developing now, though). Remember that these kiddos are capable of huge leaps and that they don't always have even development. Hang in there, slammie!
