A Google Webmaster Help thread makes you think it is about 90% accurate. Well, let me explain better.
Someone asked why has the impression count not changed from 24,900,000 in three months. Google responded that it means the impressions have not changed more than 10% over that time.
The webmaster asked:
Why are the number of impressions (24,900,000) for our web site stays exactly the same for the last three months?
Google's Asaph Zemach replied:
When you see 24,900,000 you should really think of it as 25M +/- 2.5M. Which tells you that the number of impressions has remained within plus or minus 10% for the duration you are referring to.
Pretty neat information, don't you think?
Forum discussion at Google Webmaster Help.