How could Lady Fatimah (may Allah be pleased with her) walk in the the manner of the Messenger (peace be upon him), knowing that it is forbidden for women to imitate men?

Fatimah (may Allah be pleased with her) was the most similar of people to the Prophet (peace and blessings of Allah be upon him) in her speech and walk. This similarity is either of the innate, inherited, unintentional kind, or a similarity in matters that are not exclusive to one gender over the other, such as her graceful walk and clear speech. It does not refer to the forbidden imitation, which involves one person deliberately adopting the specific characteristics of another.