2 parent disks ?

I want to emulate the MCITP 2008 network and I notice there are 2 parent disks ie. in the Base folder there is Base10A-W7-Office07-HV.vhd (8,099,552 KB) and in the Drives folder there is Win7-OFC.vhd (1,329,940 KB). What is the point of this?

I've created a fixed Base disk (20,975,616 KB)- maybe should have kept it a differencing disk-  and a child disk (6,580,224 KB) which, I suppose, I could create as another parent, but the size is far larger than the Microsoft WIN7-OFC.vhd. Why is this ?

Thanks, (Confused)James.