http://sandbox.frontline.ro/2009/09/21/plesk-howto-recalculate-traffic-and-disk-usage/
The solution: To update traffic and disk usage for all clients, run: /usr/local/psa/admin/sbin/statistics
To update traffic and disk usage for a single domain, run:
/usr/local/psa/admin/sbin/statistics --calculate-one --domain-name=domainname.com