The words lesser, lessor sound the same but have different meanings and spellings. Why do lesser, lessor sound the same even though they are completely different words?
The answer is simple: lesser, lessor are homophones of the English language.
Smaller in amount, value, or importance, especially in a comparison between two things: chose the lesser evil.
Of a smaller size than other, similar forms: the lesser anteater.
Less. Used before a participle: a lesser-known writer.
One that is lower in importance, rank, magnitude, or degree: the lesser of two evils.
One that lets property under a lease.
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").