In my experience of constantly checking stats Google results vary a lot anyway. I have had my site ranked #1 for a key search term then it would disappear to the third page the following day then back to the first page a few days later.. all from the same PC with the same IP and same Google account.
My guess is that there are clusters of servers that all have their own crawlers that sync periodically.. but this is Google black box stuff I guess.
...to answer your question I see Google Webmaster Tools as being accurate but the data never seems to be up to date. I find that it can often be a few days behind if not a week.
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Alexcruncha
FantasyBingo.co.uk
CasinoReviewSquad.com