The words cavalrymen, cavalryman sound the same but have different meanings and spellings. Why do cavalrymen, cavalryman sound the same even though they are completely different words?
The answer is simple: cavalrymen, cavalryman are homophones of the English language.
A soldier in the cavalry.
Plural form of cavalryman.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.