The words nickel, nickle sound the same but have different meanings and spellings. Why do nickel, nickle sound the same even though they are completely different words?
The answer is simple: nickel, nickle are homophones of the English language.
A silvery, hard, ductile, ferromagnetic metallic element used in alloys, in corrosion-resistant surfaces and batteries, and for electroplating. Atomic number 28; atomic weight 58.69; melting point 1,453°C; boiling point 2,732°C; specific gravity 8.902; valence 0, 1, 2, 3. See Table at element.
A U.S. coin worth five cents, made of a nickel and copper alloy.
Slang A nickel bag.
To coat with nickel.
Common misspelling of nickel.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, 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").