The words sibilants, sibilance sound the same but have different meanings and spellings. Why do sibilants, sibilance sound the same even though they are completely different words?
The answer is simple: sibilants, sibilance are homophones of the English language.
The quality of being sibilant: a hissing quality.
Plural form of sibilant.
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.