The words muscles, mussels sound the same but have different meanings and spellings. Why do muscles, mussels sound the same even though they are completely different words?
The answer is simple: muscles, mussels are homophones of the English language.
Plural form of muscle.
Third-person singular simple present indicative form of muscle.
Plural form of mussel. Cooked mussels
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").