The words rancor, ranker sound the same but have different meanings and spellings. Why do rancor, ranker sound the same even though they are completely different words?
The answer is simple: rancor, ranker are homophones of the English language.
Bitter, long-lasting resentment; deep-seated ill will. See Synonyms at enmity.
Chiefly British An enlisted soldier.
Chiefly British A commissioned officer who has been promoted from enlisted status.
Definitions 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.