Not that I’m suggesting any of these are true or even reasonable, but;
She could be adopted.
Her mother could be a squib, and her father was not Mr. Granger.
Her mother could be a squib, and she could be the result of Parthenogenesis.
Extreme luck could spontaneously cause a mutation in either gene to the magic gene, with the other parent being a squib.
They could both be wizards, unawares.
Apparently my imagination is defective.
Not that I’m suggesting any of these are true or even reasonable, but;
She could be adopted.
Her mother could be a squib, and her father was not Mr. Granger.
Her mother could be a squib, and she could be the result of Parthenogenesis.
Extreme luck could spontaneously cause a mutation in either gene to the magic gene, with the other parent being a squib.
They could both be wizards, unawares.
Apparently my imagination is defective.