The words knot, not sound the same but have different meanings and spellings. Why do knot, not sound the same even though they are completely different words?
The answer is simple: knot, not are homophones of the English language.
A compact intersection of interlaced material, such as cord, ribbon, or rope.
A fastening made by tying together lengths of material, such as rope, in a prescribed way.
A decorative bow of ribbon, fabric, or braid.
A unifying bond, especially a marriage bond.
In no way; to no degree. Used to express negation, denial, refusal, or prohibition: I will not go. You may not have any.
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.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").