Because this was hard to find…
This is an old problem that in my experience seems to affect Server 2008R2 and earlier VMs.
The fix for this is an un-documented DISKPART command: EXTEND FILESYSTEM
See below for a usage example:
DISKPART> list volume Volume ### Ltr Label Fs Type Size Status Info ---------- --- ----------- ----- ---------- ------- --------- -------- Volume 0 D Removable 27 PB Volume 1 E DATA_VOL NTFS Partition 2000 MB Healthy Volume 2 C Windows NTFS Partition 4001 MB Healthy System DISKPART> select volume 1 DISKPART> extend filesystem DiskPart successfully extended the file system on the volume. DISKPART> exitSource: https://community.spiceworks.com/t/disk-management-and-windows-explorer-show-different-sizes/681766