The words chrism, chrisom sound the same but have different meanings and spellings. Why do chrism, chrisom sound the same even though they are completely different words?
The answer is simple: chrism, chrisom are homophones of the English language.
Ecclesiastical A consecrated mixture of oil and balsam, used for anointing in church sacraments such as baptism and confirmation. Also called holy oil.
Ecclesiastical A sacramental anointing, especially upon confirmation into the Eastern Orthodox Church.
Archaic An infant wearing a baptismal robe; a baby.
A white cloth or robe worn by an infant at baptism.
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").