The words prophecy, prophesy sound the same but have different meanings and spellings. Why do prophecy, prophesy sound the same even though they are completely different words?
The answer is simple: prophecy, prophesy are homophones of the English language.
An inspired utterance of a prophet, viewed as a revelation of divine will.
A prediction of the future, made under divine inspiration.
Such an inspired message or prediction transmitted orally or in writing.
The vocation or condition of a prophet.
To reveal by divine inspiration.
To predict with certainty as if by divine inspiration. See Synonyms at foretell.
To prefigure; foreshow.
To reveal the will or message of God.
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").