The words equivalence, equivalents sound the same but have different meanings and spellings. Why do equivalence, equivalents sound the same even though they are completely different words?
The answer is simple: equivalence, equivalents are homophones of the English language.
The state or condition of being equivalent; equality.
Mathematics An equivalence relation.
Logic The relationship that holds for two propositions that are either both true or both false, so that the affirmation of one and the denial of the other results in contradiction.
Plural form of equivalent.
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.