I am trying to understand how within Google Webmaster Tools (Google Index>Index Status) I can have a figure of 2,000 total indexed pages (and falling daily) yet when I search Google using the (site:) parameter I get a consistent 23,000 pages? Also, under (Crawl>Sitemaps) it says 19,000 Indexed which is about right for our sitemap. Yes, I understand it can take days or weeks for Webmaster Tools to update but we are talking about months here! What is going on? *This site is over a year old now.
The main problem you're having is that you expect numbers put out by Google to be accurate, but they aren't. This isn't specific to Google, it's true for all major web search engines.
The total result count is just an estimate, and it's easy to see:
The numbers published by Webmaster Tools are probably more true than the total result count, but, they are reported with a delay and probably don't mean what you think they mean.
Finally, a comment by @w3dk suggested "jumping through the SERPs" to find out how many results there really are. That is not accurate; all searches on Google return around 500-700 results even if it claims there are millions or 10s of millions of total results.