Tsarinas named Feodorovna 

Can some Russian expert explain why the significance of the apparent patronymic Feodorovna which was given to several tsarinas of Russia on their marriage and conversion to Orthodoxy, even though their fathers were not named Feodor. According to the disambiguation pages for Alexandra Feodorovna and Maria Feodorovna, this name was given to TWO tsarinas named Alexandra, most famously the last tsarina whose father's name was Louis, and TWO named Maria, including the second-last tsarina whose father was King Christian IX of Denmark. Why were all four of these tsarinas given the patronymic Feodorovna which seems to imply daughter of Feodor? Who was Feodor??