Incorrect percents of disk usage displayed in Statistics tab. When disk quota is overused Plesk always show 100%, instead of 110% etc. At the same time in e-mail report overusage is provided properly, 110% etc..
Bug #PPPM-2034. It is fixed in Plesk 12 MU #63 and Plesk Onyx.
Install the latest microupdates or upgrade to the latest Plesk version.