The words straighten, straiten sound the same but have different meanings and spellings. Why do straighten, straiten sound the same even though they are completely different words?
The answer is simple: straighten, straiten are homophones of the English language.
To make or become straight or straighter.
To make narrow.
To enclose in a limited area; confine.
To put or bring into difficulties or distress, especially financial hardship.
Archaic To restrict in latitude or scope.
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.