I've seen this issue a few time in the past with virtual as well as physical systems. It looks like the partition information has not been written properly, i.e. the partition size is correct (20GB), but the partition end has not been updated for whatever reason, thus the old size in the explorer. What usually solves the issue is to slightly increase the virtual disk's size again, and repeat the resizing. Btw. I usually use a Windows 7/2008 ISO image to resize the partition. Just boot the VM from the ISO image to the point where it shows the language selection, then press Shift-F10 for a command prompt and use the diskpart utility to "extend" the partition.
André