I think the only time that Google changes a word in quotes is if the spelling algorithm is very confident. For example, if you searched for [barack obama "autobiolgraphy"], you are going to get results for "autobiography" with the option to really search for "autobiolgraphy".

Other than that, I'm pretty sure that a word in "quotes" must appear in the result to be retrieved, without any alternatives. If you have a counter-example, I would love to see it! (I work on that code.)