The words fin, Finn sound the same but have different meanings and spellings. Why do fin, Finn sound the same even though they are completely different words?
The answer is simple: fin, Finn are homophones of the English language.
A membranous appendage extending from the body of a fish or other aquatic animal, used for propelling, steering, or balancing the body in the water.
Something resembling a fin in shape or function, as:
A fixed or movable airfoil used to stabilize an aircraft, missile, or projectile in flight.
A thin, usually curved projection attached to the rear bottom of a surfboard for stability.
A native or inhabitant of Finland.
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.