You have a bit of a challenge with your Vicidial version listing. There is a "Vicibox" version (4.x.x or 5.x.x) and a "Vicidial" version (2.x or 2.x.x). There was no "4.0" if I recall correctly. So see if you can find out which 4.x.x you have and note that this is the ViciBOX version (installer) followed by the ViciDIAL version (Dialer!).
You have a fairly old and "interim" version (we have NO clients on 2.6! plenty on 2.4 and 2.8, even a couple still on 2.2 and ONE on 2.0.5, but No One on 2.6! LOL). That being said, you should try adding "&DB=1" to your URL, this will often show you the DB queries that generated your data. The first bunch will be security to pull the page and such, but eventually, if you're lucky and show the "source" for the page, you may get the real queries. This will tell you which tables the data is from and give you a hint of what is happening.
I'm not saying vicidial reports are perfect (especially on that version), but I have heard this question several times and usually it turns out that the data sets are different. For instance, agents transfer calls between them, resulting in two "agent records" for each "call". Many things are possible, and looking at the underlying data helps. Also helps to test call on a Sunday (no other log entries!) and see what happens with those isolated test calls.
Happy Hunting!