I checked, and advanced search gets the same results, with default settings, as basic search. So you can look at advanced to see what settings it's using. However, it doesn't help much in answering your question.
I tried searching for "gkrsvyelm" which is, (at least as far as I know!), a meaningless string. I still got loads of results, but obviously none where I could see why they were included, except that one or two contained other meaningless strings (perhaps being the words most like my string). So I'm pretty sure the relevance search does its best to find something close, whatever you enter.
If you search "fire stick", i.e. with a space, you do get results that make a basic kind of sense, i.e. they contain both words, reasonably close together, albeit they concern fires and sticks as opposed to Amazon products. Pure speculation, but it appears to do better at relevance if it's dealing with common words, which would give it a larger set of records to go at. Whereas I tried "pecking", as in "pecking order", as something that might appear only once or twice, and got equally strange results as for "firestick". Actually Google suggests that "
pecking" is significantly more common than "
firestick" in YACF.
If you search "Amazon firestick" then results for Amazon overwhelm those for firestick. I think that, somehow, the search does better with words that are common in the forum. It may simply be faiing to index some threads. But it may even set to discount words that occur less than N times (or N+1 in the case of a cycling forum). It may also not recognise that "firestick" and "fire stick" may be closely-related strings, instead treating words as quite separate because one is not an obvious stem of the other.
These are more vague and speculative musings than answers, sorry.
