The words marten, martin sound the same but have different meanings and spellings. Why do marten, martin sound the same even though they are completely different words?
The answer is simple: marten, martin are homophones of the English language.
Any of several principally arboreal carnivorous mammals of the genus Martes, related to the weasel, mainly inhabiting northern forests, and having a slender body, bushy tail, and soft fur.
The fur of one of these carnivorous mammals.
Any of various swallows, such as the house martin or the purple martin.
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.