This is a specifically French construction: http://french.about.com/library/weekly/aa032500.htm
The following is the information provided by Duolingo [under 'Explain' when one clicks on 'C'est' in this sentence]:
C'est instead of il/elle est C'est is used when the noun has an article (le, la, l', un or une) or a possessive before it. Say: C'est ma femme, not: Elle est ma femme.
No, it means that c'est une fille is the only correct way to express this idea. It is all according to French linguistic rules.
Here are two links on this matter, the first in French, the second in English: http://www.academie-francaise.fr/il-est-cest-un-0 and http://www.frenchtoday.com/blog/cest-versus-il-elle-est