How is a unique visitor counted in Piwik PRO?
To define unique visitor Piwik PRO is using first-party cookies which are being stored in your visitor browser. Once the same person comes to your website on two or more different devices then Piwik PRO is going to identify two unique visitors.
If the visitor has disabled, deleted or blocked cookies then a simple rule of thumb is being used to try to match the visitor to a previous visitor with the same features (resolution, browser, IP, etc.).
This method called “Fingerprint” is, unfortunately, less accurate and may be especially problematic in case of a standardized environment where a lot of identical machines use the same IP addresses (they can all be identified as a single visitor).