Hi,
What version of VC do you have? I'm pretty sure that in earlier versions the % value being shown for the alarm is incorrect is actually measured as KBps by VC (this is fixed in VC2.5U2/U3) so the way the alerts are set means that although it looks like 80% usage it's actually triggered when you go past 80 KBps of usage, for example.
Hope this helps
DB