Hi! This is my first time posting here, please be gentle with me

I installed Backtrack 5 (32-Bit) for several weeks now to use OpenVAS and the Greenbone Security Desktop to try and assess the vulnerability of a fairly sizable private network containing several servers.

Everything seems to work well, I followed the instructions I found in the BT-Wiki and found some vulnerabilities. The odd thing is this: changing nothing at all, no system updates, no NVT-Updates, no changes in GSD, running the exact same task on the exact same targets, I get different results. The number of high vulnerabilities is all over the place... running the same scan four times without touching the servers or the software, I get four different numbers of results.

The results aren't off by just a little bit, either. They almost doubled from one scan to the next only to go back down .... and rise... all this over a timespan of roughly 5 hours.

My question: wtf?

I can't find anything about what might be causing this. Can anybody point me in the right direction?