Well if I may chime in here....

Feed stores tend to source their chicks from either local breeders or hatcheries. My guess is she came from a show breeder that
probably didn't segregate his colors into different breeding pens. Maybe that is why they sold to the feed store. But
Anywho, I digress.
I speculate that she is indeed a Dutch Bantam but possesses a few key color genes.
Certainly, she has the mottle gene in her. But, not quite the normal mottle gene. I believe she has what is called "hysterical mottling" which is the same form found in Anconas and Exchequer Leghorns.
I also believe that, like a Golden Neck D'Uccle, she has 1 copy of dominant white.
So, what this would
theoretically make is a hysterically mottled bird with the dull buff and white speckling/filling of a golden neck.
If you breed her to a Mille Fleur I would expect to her to throw 50% MF chicks and 50% Golden Necks.
Hope this helps