The words em, m sound the same but have different meanings and spellings. Why do em, m sound the same even though they are completely different words?
The answer is simple: em, m are homophones of the English language.
Printing The width of a square or nearly square piece of type, used as a unit of measure for matter set in that size of type.
The letter m.
Printing Such a measure for 12-point type; a pica.
The 13th letter of the modern English alphabet.
Any of the speech sounds represented by the letter m.
The 13th in a series.
Something shaped like the letter M.
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.