The words canvas, canvass sound the same but have different meanings and spellings. Why do canvas, canvass sound the same even though they are completely different words?
The answer is simple: canvas, canvass are homophones of the English language.
A circus tent.
A heavy, coarse, closely woven fabric of cotton, hemp, or flax, used for tents and sails.
A piece of such fabric on which a painting, especially an oil painting, is executed.
A painting executed on such fabric.
To examine carefully or discuss thoroughly; scrutinize: "The evidence had been repeatedly canvassed in American courts” ( Anthony Lewis).
To go through (a region) or go to (persons) to solicit votes or orders.
To conduct a survey of (public opinion); poll.
To make a thorough examination or conduct a detailed discussion.
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").