While using Nginx as a reverse-proxy, IP address of the server, not client is written to the Apache server log. And
PHP $_SERVER["REMOTE_ADDR"] function returns wrong output.
Therefore AWStat statistics is not properly calculated.
The issue is considered to be a software issue with id #PPPM-2144.
The issue has been fixed in Plesk 12.0.18 MU#24. Please install all the microupdates.
For Plesk 12.5 refer article #127201