When the paper compared some actual website numbers here is what they came up with:
The paper authors, Dick Bennett of ImServices Ltd, Steve Douglas from DJG Marketing, Bruce Rogers from Forbes, and Gerard Broussard from Pre-Meditated Media raise a big question of just how accurate the traffic numbers that these services provide might be.
The paper also shared a "Consumer Reports" style look under the hood of these services, showing detailed and contrasting methodologies being used by each. I recommend you read a review of the methodology of any service whose numbers you using:
Download and read the entire paper: http://www.readershipsymposium.org/online-measurement-%E2%80%93-too-many-numbersrelationships