The words leaven, levin sound the same but have different meanings and spellings. Why do leaven, levin sound the same even though they are completely different words?
The answer is simple: leaven, levin are homophones of the English language.
An agent, such as yeast, that causes batter or dough to rise, especially by fermentation.
An element, influence, or agent that works subtly to lighten, enliven, or modify a whole.
To add a rising agent to.
To cause to rise, especially by fermentation.
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").