I find it really frustrating that I cannot rely on the Google API to return an estimated number of results that is even remotely similar to the number of results returned by a standard Google web search on Google.com.
Thus, I can't rely on the Google API for tracking the number of pages of my site that is indexed over time. Or the number of back links reported.
I'm sure plenty of people would rather not scrape Google SERPs to track their indexation or link popularity numbers, but Google, you give them little choice!
If you want fewer SEO tools scraping your pages for the results numbers, then I suggest making the API return the same number of results that Google.com does.