The words bat, batt sound the same but have different meanings and spellings. Why do bat, batt sound the same even though they are completely different words?
The answer is simple: bat, batt are homophones of the English language.
A stout wooden stick; a cudgel.
A blow, such as one delivered with a stick.
Baseball A rounded, often wooden club, wider and heavier at the hitting end and tapering at the handle, used to strike the ball.
Sports A club used in cricket, having a broad, flat-surfaced hitting end and a distinct, narrow handle.
Pieces of fabric used for stuffing; batting.
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").