In checking Plesk Stats, the traffic is much lower that the actual traffic.

Plesk Web Stats button only links to now SSL traffic. See: https://support.plesk.com/hc/en-us/articles/213951505-Different-statistics-values-for-SSL-and-non-SSL-traffic

As a workaround, SSL traffic statistics can be accessed using the following link:

https://example.com/plesk-stat/webstat-ssl

FTP statistics can be accessed using the following link:

https://example.com/plesk-stat/ftpstat
https://example.co/plesk-stat/anon_ftpstat