I am a little bit confused about number of visitors on my site, three stats counting programs show different numbers, I am not sure which is the closest to the real number.
Google analytics and Piwik’s stats are based on JS code execution, AWstats gets data from apache logs, Image Statistics Addon for Pixel post probably records hits when database lookup is performed.
Look at your apache logs. Grep for a specific URL for a given date and then manually calculate your results.
For each IP address, look at the user agent string and what other URLs are fetched. Is the user agent string and surfing behaviour consistent with a human visitor, or does it look like a bot?
A cursory look at pixelpost suggests one area for under-reporting is where the DOM creates page content dynamically (e.g., iframes, anchor links, etc). You’ll have to add the tracking code or attach listeners to links, as appropriate, e.g.,