Quota exceeded but not displaying more than the limit

I have several virtal servers which suddenly exceed there quota limit, but when I look in the Virtualmin Adminstration, it show less than the limit.

When I change the limit by increasing it, then all works fine again.

The last two had the following limits and displayed quota used

Limit : 5 GB Quota used : 1.5 GB.
Limit : 2 GB Quota used 1.4 GB.

The server is
Operating system Ubuntu Linux 8.04.4
Webmin version 1.540
Virtualmin version 3.84 Pro
Theme version 8.0
Time on server 23/May/2011 10:03
Kernel and CPU Linux 2.6.24-26-server on x86_64
Running processes 239 [Running processes] Real memory 1.97 GB total, 1015.69 MB used [Memory used]
Virtual memory 3.29 GB total, 4.03 MB used [Swap space used]
Local disk space 76.10 GB total, 33.61 GB used [Disk space used]
Package updates 58 updates to system packages are available. Use the Virtualmin Package Updates module to install them selectively.

Disk /dev/sda: 85.8 GB, 85899345920 bytes
255 heads, 63 sectors/track, 10443 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00049d85

Device Boot Start End Blocks Id System
/dev/sda1 * 1 31 248976 83 Linux
/dev/sda2 32 10443 83634390 5 Extended
/dev/sda5 32 10443 83634358+ 8e Linux LVM

What to do? Because I get no warning that we are reaching the limit.