I saw a question I wanted to answer but had just closed the page, so I looked for it using search. I typed a line from it exactly and used quotation marks, but it came up with loads of questions only slightly similar.
Probably because the way the site search was designed was not to match exact phrases found in a question. If I were take a guess I would say that common words like "the", "we", "us", "I", etc are all filtered out because they are too generic (all questions contain them). Next it would identify the keyword(s) you're looking for and find all the topics that contain those keywords. Once the results are found it probably ranks them by popularity of some sort; whether that is the question rating, the number of responses or something else altogether only the site creators can tell you for sure.
As Dr_fix has suggested, try a Google search if you know the exact wording of part of the question.