The words pupal, pupil sound the same but have different meanings and spellings. Why do pupal, pupil sound the same even though they are completely different words?
The answer is simple: pupal, pupil are homophones of the English language.
Of or pertaining to the pupa stage of insect metamorphosis.
A student under the direct supervision of a teacher or professor.
Law A minor under the supervision of a guardian.
The apparently black circular opening in the center of the iris of the eye, through which light passes to the retina.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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").