The words knobs, nobs sound the same but have different meanings and spellings. Why do knobs, nobs sound the same even though they are completely different words?
The answer is simple: knobs, nobs are homophones of the English language.
Plural form of knob.
Plural form of nob.
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.