The words sleave, sleeve sound the same but have different meanings and spellings. Why do sleave, sleeve sound the same even though they are completely different words?
The answer is simple: sleave, sleeve are homophones of the English language.
Archaic A fine thread or skein of thread.
A part of a garment that covers all or part of an arm.
A case into which an object or device fits: a record sleeve.
To furnish or fit with sleeves or a sleeve.
up (one's) sleeve Hidden but ready to be used: I still have a few tricks up my sleeve.
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").