Why is the main character, Mathilde Loisel, described as someone with a "mistake of destiny"?

M. Loisel believed her destiny was one of wealth, grandeur, and leisure. The story describes her as having been "born, as if by an error of fate, into a family of clerks." This simply means that her aspirations were far above her social status.

M. Loisel desired to be one of the wealthiest family in town but unluckily she belong to a middle class family.