There are many different sources for web analytics. You can set up free Google Analytics on your site, you might get analytics from your website or blog host, and you might have paid for an advanced analytics tool.
But what happens when they all return different numbers in their reports? Which one is right?
We are not our devices
First of all, nothing is 100% accurate and precise. None of the analytics packages tracks exactly how many humans are visiting your site, reading your pages, and clicking on your links. Analytics tools track visits from computers, not necessarily distinguishing between who is using the computer to access your site. Different people can use the same computer, and a single person can use different devices to access your site. And if a visitor takes a phone call while browsing your site, the time they spend on a particular page can be miscalculated.
Reports that tally the keywords that visitors use to find your website are defined differently by different analytics tools. Google Analytics includes counts for search engine queries and paid keywords in the single "keywords" count. Google Webmaster Tools displays only the most significant words Google found on your site. And some tools round the results to one or two significant digits, which can make a big difference in lightly visited sites. And Webmaster Tools shows only search queries from Google, whereas your other tools may combine search results from all search engines.
Are analytics useful?
Each analytics tool tracks the data differently, tallies up the data differently, and reports differently. If you compare the visitor numbers tracked by Google Analytics to those tracked by your web server, the Google number is almost always a smaller number. But while the numbers aren't exact, the data is still very useful. You can compare sets of data, look for bigger patterns and trends, and combine data to tell the complete story. You can read more about gaining insights through comparisons in "The Right Way to Use Analytics."