There could be a various number of reasons why Piwik numbers are different from your other web analytics solution.
- Tracking method: Other softwares (such as AWStats, Webalizer, Analog, etc.) or custom server-side script would record all traffic, including search bots or spam bots. Check that you are comparing Piwik with a similar web analytics tool
Concerning Google Analytics, several users compared Piwik and Google Analytics numbers, and found very similar statistics: on significant traffic, differences were always less than 5 percent.
Conservative counting is definitely best – it makes your statistics more realistic on what you are trying to measure. Most analytics packages are used to measure relative upswings, we recommend using the same tool for all measurements and relative comparisons.