The words tenser, tensor sound the same but have different meanings and spellings. Why do tenser, tensor sound the same even though they are completely different words?
The answer is simple: tenser, tensor are homophones of the English language.
comparative form of tense: more tense
Anatomy A muscle that stretches or tightens a body part.
Mathematics A set of quantities that obey certain transformation laws relating the bases in one generalized coordinate system to those of another and involving partial derivative sums. Vectors are simple tensors.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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").