Awstats and Plesk counts are based on information from your webserver which counts the size of the data only. This does not take into account the overhead bytes in each packet header, but these bytes are counted by your webhost.
Furthermore there is incoming traffic which you should take into account. These are not only the http requests coming from client computers, but also acknowledgment packets on the lower TCP/IP levels which are not counted by your server side stats packages.
And then there is traffic other than the regular http traffic. Phranque already mentioned backup traffic, but there is also traffic of other protocols like DNS name resolve requests, SMTP traffic if you have email software enabled, and all kind of traffic of hackers, spammers, port scanners, who probe your servers without you being aware of.