The words petrel, petrol sound the same but have different meanings and spellings. Why do petrel, petrol sound the same even though they are completely different words?
The answer is simple: petrel, petrol are homophones of the English language.
Any of numerous black, gray, or white sea birds of the order Procellariiformes, especially the storm petrel.
Chiefly British Gasoline.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.