It does seem that the numbers change for different numbers of results, but as Russ mentioned, they are indications/estimates.
What I'd be more interested in is which pages Google Webmaster Tools knows about from your sitemap.xml (since these are the pages that you consider important). With a larger site you could also split your sitemaps into sections to give easier analysis of which sections are poorly indexed.
For more resources on URL parameters:
http://searchengineland.com/google-power-user-tips-query-operators-48126
http://searchengineland.com/google-power-user-tips-serp-url-parameters-49736
