In my niche the most popular KW is 'X' If you search 'Y' google will ask "did you mean X?"
The SERPS on the for 'Y' show the variation of the KW (X, ,widgetion, widgeters) bolded. But no 'Y' on the SERPs page. If the search is done again for Y in quotes the SERPs will contain pages that have Y in them.
So google is not really returning results for Y when searched, because those results do exist. And the results they do show are not exactly the same as the SERPs for X. So they are not those either.
What is Google showing for the results when you see did you mean...
Someone else take this up if i am woring but ... what It does is look at the term you have typed and if it hasent seen the term before or there are 1mill of a term with a similar spelled wrod it gives you that option.
It is to keep the idiots happy who cant type the word widgte
It is basically showing you how ignorant you are and being a pedant about spelling