A WebmasterWorld thread reports Google bolding or highlighting words in the search results that are not actual words you searched for. Rather, Google will bold synonyms of the keywords you searched for.
Searching for [leather couches] will return Google bolding words like “sofa” as well. Here is a picture:
Honestly, I really didn’t think this was new Google behavior but the folks in the WebmasterWorld forum saying it is and sharing other search terms which trigger this.
In fact, searching for [couches] won’t result in Google bolding “sofa” but searching for [leather couches] does. Why?
Forum discussion at WebmasterWorld.
