The words passable, passible sound the same but have different meanings and spellings. Why do passable, passible sound the same even though they are completely different words?
The answer is simple: passable, passible are homophones of the English language.
That can be passed, traversed, or crossed; navigable: a passable road.
Acceptable for general circulation: passable currency.
Satisfactory but not outstanding; adequate: The actors gave passable performances but the singers seemed unrehearsed.
That can be legislated: a passable bill.
Capable of feeling or suffering; sensitive: a passible type of personality.
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").