We all know that females have an inactivated X chromosome (Most of its genes are inactivated). Consider a female who is a heterozygous carrier of an X linked abnormality, If the inactivated X was that carrying the normal gene, shouldn't the abnormality be expressed in this female? Why is it not expressed even though the other X is inactivated?
Because in different cells different copies of the X chromosome might be inactivated. So, in (approximately) 50% of the cells you will have the X carrying the normal gene activated, and in the remaining 50% you will have the other. You might start reading something here, and then go in further detail to understand. A nice article is here