One domain and the first two keyword combinations are free. If you're satisfied with rankometer you can purchase an unlimited access license.
The official Google API comes with certain restrictions which apply to all apps using it. It returns up to 64 results for each keyword query. In addition, you're only allowed 100 requests per day. Rankometer informs you if anything went wrong with the Google part.
The results returned by Google should be similar to the ones you would see in a browser. There have been reports about distorted results in some cases, and Google has confirmed that the API does not necessarily reflect the results of a browser based search.
Anyway, you'll never get closer to the real results than with the official Google API.
Rankometer uses this API as the only legal way to automatically fetch search results from Google.