T3500 Memory Configuration

I just bought a used t3500.  It has 12 GB of ram.

It has a configuation that is not listed in the manual.

Slot 1 has 4GB, Slot 2 has 2GB, Slot 3 has 2GB and Slot 4 has 4 GB.  As labeled on the board and verified though the bios.

When looking at it it looks like 4, 4, 2, emtpy 2.

Anyways this is not listed in the manual as a supported config, but it is working

What other configs work and why does this one.  It seems crazy...

Thanks!