The words impassable, impassible sound the same but have different meanings and spellings. Why do impassable, impassible sound the same even though they are completely different words?
The answer is simple: impassable, impassible are homophones of the English language.
Impossible to pass, cross, or overcome: impassable roads; impassable problems.
Not subject to suffering, pain, or harm.
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.