The words gnatty, natty sound the same but have different meanings and spellings. Why do gnatty, natty sound the same even though they are completely different words?
The answer is simple: gnatty, natty are homophones of the English language.
Neat, trim, and smart; dapper.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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.