The words canvases, canvasses sound the same but have different meanings and spellings. Why do canvases, canvasses sound the same even though they are completely different words?
The answer is simple: canvases, canvasses are homophones of the English language.
Plural form of canvas.
Third-person singular simple present indicative form of canvas.
Plural form of canvas. (UK spelling)
Plural form of canvass.
Third-person singular simple present indicative form of canvas.
Third-person singular simple present indicative form of canvass.
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.
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").