Google Scholar Syntax quirks: results depend on order of search strings
If you want to use google scholar as a means of measuring the number of papers on a certain topic: don't.

Contrary to what we often see in reviews or other publications, you did some more research than simply copying the number of results from a naïve search, and that research indicated that this number is highly unreliable (as should have been expected). Conclusion: Google scholar is not the right tool for the job. Its algorithms are unknown, and the sources it searches are unknown as well. Maybe it even counts the same paper multiple times, depending on which words you search for, who knows?

Having said that, in practice nobody really seems to mind an introduction stating "a Google search for XXX returned N results", but the question remains how useful such a statement is, given your findings.