The words mantel, mantle sound the same but have different meanings and spellings. Why do mantel, mantle sound the same even though they are completely different words?
The answer is simple: mantel, mantle are homophones of the English language.
An ornamental facing around a fireplace. Also called regionally mantelpiece.
The protruding shelf over a fireplace. Also called mantelpiece, mantelshelf; also called regionally fireboard.
A loose sleeveless coat worn over outer garments; a cloak.
Something that covers, envelops, or conceals: "On a summer night . . . a mantle of dust hangs over the gravel roads” ( John Dollard).
Variant of mantel.
The outer covering of a wall.
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").