If you do a search through say Google you do get far more than you would get using a SOL search even if you are not a Premier member.
That's not true at all.
Google/Search Engines tend to show you chapters of the same story as additional results.
Google/Search Engines are not a reliable way to search inside SOL. It may be a way around requiring a premier membership to search inside the story's contents, but it doesn't deliver complete results; nowhere near complete.
The main reason for that is that we don't allow Google/Search Engines to index everything on SOL.
Whatever you can see when you're not logged in on SOL is what Search Engines can see and index. So whatever has a preview, may be indexed by an external search engine. For single part stories the preview is the first 5000 characters of the file.
For example, the site's database holds data for 150,480 Chapters (for 13,000+ multi-part stories), plus 24,528 single-part stories. Google's search console (accessible to the webmaster) reports 42,426 pages from SOL (including blogs, this forum, various listings pages etc...) in Google's index. That means google indexes less than 20% of Storiesonline's contents.