However.. this accuracy is only true if you have Google Analytics installed CORRECTLY on your site. This means the tracking script should be installed only one time, and in the correct location on the page. If you have duplicate versions of the tracking code or have it installed too early in your head section, you can get falsely inflated analytics.
To demonstrate just how dramatic this inflation can be, I did an experiment on my own site. I installed a second tracking code into my html and then watched my traffic over the next 4 days. This is what happened: