Why would a Hyper-V VHD still show 100% disk space used after adding 20 Gig?

Hyper-V Host is 2012 R2 with several guests.

My primary guest is a TFS server using a 100G VHD. I wasn't really keeping track of it that close and at some point the VHD hit 100% space used. I expanded the size by 20G, but the space used remained at 100%.

So I begin trying to understand how to fix the issue by asking why did the 20G I added not leave me with open space on the VHD?

You added space to the VHD. You didn't, however, change the partition table on that disk so that the file system on the disk can use the extra space. So, to the guest OS, it looks like a disk with one partition and some empty space that could be used for another partition. You can either add another partition (which would be an odd choice) and then format it with a second file system, at which point you'll see two drive letters (if you're using them) and all the space will be available. Or, more usefully, you can extend the partition that exists, at which point the existing file system will use the extra space.

Did you extend the disk in the Disk Management console in the guest? If not then that's why. You need to first expand the VHD in the settings of the VM and then you need to extend the disk in the guest Operating System.