yelp is always confusing, i'll try to clear up stuff.
first is, yelp has the famous yelp filter
basically tries to filter out crap and fake reviews, you can see all filtered reviews if you scroll all the way down on a restaurant review page and theres a link
"[number] other reviews that are not currently recommended"
i am pretty sure advertising with yelp doesn't affect this filter.
advertising with yelp means that a user will be on the yelp ad/website and type in "chinese ny ny" or whatever and you will be the first result with "AD" at the front of your name. these are either by ppc or cpm, pay per click of usually 3bucks a click or 3bucks per 1k impressions, what is un-heard of is that yelp locks a business in with a contract of min ad spend (usually 500ish) and early temination fees.
again, i'm sure the yelp filter isn't affected by advertising with yelp, if they are, then it is lying/overzealous yelp reps who are trying to pimp yelp out to small businesses in order to meet their quota or get a commission. (this also gets you the yelp sticker on the store door)