Thou art Cephas, and upon this Cephas, I will build my Church.
Thou art Rock, and upon this Rock I will build my Church.
Thou art Peter (a man’s name that means “Rock”) and upon this Rock I will build my Church.
Thou art Petros (a man’s name that means “Petra”) and upon this Petra I will build my Church.
If I discern that my baby daughter’s Patron Saint is St. Peter, I don’t call her “Peter,” because “Peter” is a boy’s name. Instead, I call her “Petra” which is a feminine name that means the same thing as “Peter.” Little Petra understands, by the time she is three years old, that “Petra” means the same thing as “Peter,” and that this is how she is named after St. Peter. She understands perfectly that St. Peter is a man, and that she is a girl, and that this is the reason for the slight difference in pronounciation, of the same word.
In the same way, St. Peter (who was being called “Cephas,” at the time) also understood that when St. Matthew was writing down his story, that St. Matthew could not write “Petra” for his name - instead, St. Matthew invented a new word - “Petros” - which signified a male name meaning the same thing as “Petra.”
Little Petra “gets” it - it’s not really all that complicated.