Phrase “ABC” will never match something like “AB” or “BF” – that is not possible, the phrase must be contained somewhere (at least partially), but not the other way around (like the content words match the phrase partially). The match is somewhere else, like in a taxonomy term, custom field etc.. It can be a partial/fuzzy match too.
By default the results are ordered by the relevance, make sure to check that here. Then, you can try experimenting with more restrictive search logics. I also recommend using the index table engine for the best possible relevance matches.
