My gut reaction says a lot about my gut, too.
A kindergartner spontaneously ready for 3rd grade math is much more uncommon than 2:80. And most parents won't teach their 5yos multiplication and division (other than as a one-off - I once demonstrated long division to DD-then-in-first as a tool for factoring, but she clearly wasn't ready for 4th or 5th grade work in general), even if the kids are capable and interested. Whereas they will let their kids read harder books without even thinking - I was shocked to see that the Rainbow Fairies books are 4th grade level in AR!
I'd say it's likely there's at least one other kid in K who would be able to do first grade math, although writing skills might limit any advanced work to oral practice / manipulative work.
As she gets older, it becomes more likely that fine motor skills will be less of a hindrance to otherwise-advanced kids, it becomes more likely that some of her age-mates will have a sudden skill-spurt in math - and it becomes more likely that she'll be farther and farther ahead *anyhow* due to pacing issues, if you're continuing to enrich her at home.
My DD is up a grade from her agemates in math, but a fair percentage (I agree with Grinity's 15% estimate) of her agemates could have handled being a grade up in math. A lot of that has to do with the grades in question - in our district, 2nd is mostly review of addition and subtraction facts. An end-of-2nd grader who hasn't been exposed to significant multiplication and division practice at home is unlikely to be spontaneously ready for 4th grade math in the fall.
