Let's start with this. No two metrics programs measure exactly the same things in exactly the same way. Therefore they will never agree.
Hits, Page Views, Unique Visitors
What is a hit? In web analytics, a hit is any request for a file from a web server. By request means a hit calculates page content delivered, all images to complete that page, and any additional files that need to be loaded to make the web page you are looking at, appear the way it does.
What is a page view? A page view is a request to load a single page of an internet site that results from a page request from a web surfer clicking on a link on another HTML page which is pointing to the page in question.
What is a unique visitor? A unique visitor is access from a single IP to a web server that generates page views and hits during a particular visit. When a visitor has cookies disabled, there is no way of establishing if they are a unique visitor or not.
Note this about wordpress stats:
The following are not counted:
* Visits from registered users of the blog when they are not logged in.
* Visits to uploaded documents and files.
* GoogleBot and other search engine spiders.
Are you savvy to why Quancast exists and how much reliance to place on it?
Quancast measures metrics for those who need to know what those figures are they are deriving income form their sites through advertising, affiliate schemes an PPC. WordPress.com is deriving income form advertising on our blogs so I would NOT dismiss those metrics from Quantcast.
Sitemeter, Statcounter, wordpress stats and all the others will never agree. Each one of them decides how and what they will count as a hit. Some count page views and some count unique visitors. Therefore, use any of the stats counters only as a general guide to hits.
Understand that an application that is not running on the same servers your blog is on is going to be susceptible to wild fluctuations. This is because all hits have to be transferred over the internet to different servers, and there are literally thousands of things that can go wrong between the server your blog is on and the server at the stats place.
Also be aware of the possibility that the software or hardware at the stats place may be broken and not recording, or counting things as intended.
Just to be clear about where I am at I am not stats obsessed and I freely admit that I don't even like discussing this topic at all. I don't have time time I wish I had to blog in so you can bet I'm not fixated on blog stats --- I have our business book to stare at.
I disagree with raincoaster on Alexa. I find their data to be very useful but I'm not into fighting so I will leave it at that.