The words scull, skull sound the same but have different meanings and spellings. Why do scull, skull sound the same even though they are completely different words?
The answer is simple: scull, skull are homophones of the English language.
One of a pair of short-handled oars used by a single rower.
A long oar used at the stern of a boat and moved from side to side to propel the boat forward.
A small light racing boat for one, two, or four rowers, each using a pair of sculls.
To propel (a boat) with a scull or a pair of sculls.
The bony or cartilaginous framework of the head of vertebrates, made up of the bones of the braincase and face; cranium.
Informal The head, regarded as the seat of thought or intelligence: Use your skull and solve the problem.
A death's-head.
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").