If someone searches for a word or term with or without a hyphen, Google can show different search results based on the query having a hyphen in it or not. So for example, [popup-style] vs [popup style] or [s-eo] vs [seo]. The question originally came up in a Moz Community thread, which no one had a solid answer for so John Mueller from Google was asked about it on Twitter.
The webmaster said the site ranks on the first page of Google's search results for the keyword without the hyphen, but when you add the hyphen in the keyword, it doesn't rank.
John Mueller said it depends, "sometimes they're seen as reciprocal synonyms, sometimes not." So in some keyword cases, Google would show the same results for a keyword with and without a hyphen and other times, it won't.
We don't special-case "-", sometimes they're seen as reciprocal synonyms, sometimes not. Eg "S-EO" is not really "SEO"— John â.o(â§â-½â¦)o.â (@JohnMu) June 7, 2017