Google has a good reason to not trust optimized sites.
If I were to sell contact lenses I might decide to go after the term "Cheap Contact Lenses Online". I'll optimize the begeezes out of my site for that one keyword. At the end of the day, Google ranks me #1 for a four word phrase. Not too hard, but Google finally decided I'm the most relevant for that term. The only problem is that my site sells each pair of contact lenses for $3000. Cheap, right?
While it has impacted a number of words, I'm seeing a growing number of adjectives being filtered out by Google.
To survive, Google has to continue to bring back the best results. If not, Bing, Yahoo, or the next best thing might come up and take away search share. By limiting adjectives, like "cheap" or "discount", Google is able to provide searchers with more respectable results. Numerous ecommerce sites will use these adjectives regardless if they're actually the cheapest and now Google is starting to cut this variable out of their algorithm.
Adding a plus sign before your search disables this specific tip feature.
This raises the question, Why is this tip at the bottom and not at the top of results like most other tips? Two quick answers are:
1. Google just doesn't want to tell you that you're not getting what you requested. 2. It's not a misspelling or error, so there's no reason to make you refine your search (they did it for you).
Terms have also been shown trimmed down from their original query.
While it is still an adjective, it is not as much of an extreme. The results for this query could be some very hot online deals, but chances of them being THE hottest online deal are not very likely. The shoes showing up in SERPs may be cheap, but it is highly unlikely they are THE cheapest.
This raises another question, Why doesn't this show up for every search? If shopping results appear in the SERPs, this tip is not as necessary as searchers can make a more accurate assessment from the 3-5 shopping results. Beyond that, it could be based on the number of search results containing that adjective in the title tag, it could be based on the popularity of that search, it could be any number of things.
This is just another interesting algorithmic twist impacting ecommerce sites as well as other industries. Google doesn't trust what you have to say and in this latest step started taking that out of the equation.