The words caster, castor sound the same but have different meanings and spellings. Why do caster, castor sound the same even though they are completely different words?
The answer is simple: caster, castor are homophones of the English language.
One that casts: a caster of nets.
A small wheel on a swivel, attached under a piece of furniture or other heavy object to make it easier to move.
A small bottle, pot, or shaker for holding a condiment.
A stand for a set of condiment containers.
An oily, brown, odorous substance obtained from glands in the groin of the beaver and used as a perfume fixative.
A hat made of beaver fur or an imitation.
A heavy wool fabric used especially for overcoats.
Variant of caster.
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.