The words cygnet, signet sound the same but have different meanings and spellings. Why do cygnet, signet sound the same even though they are completely different words?
The answer is simple: cygnet, signet are homophones of the English language.
A young swan.
A seal, especially one used officially to mark documents.
The impression made with such a seal.
To mark or endorse with a signet.
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.